1.5: Primary Treatment
Learning Outcomes
- Describe different methods to measure wastewater flow
- Examine the purpose of primary treatment
- Compare and contrast conventional primary treatment with chemically enhanced primary treatment
- Assess how specific gravity and density relates to the removal of solids able to settle
Flow measurement
After preliminary treatment, and all of the large debris and inorganic solids are removed, it’s vital to measure the amount of wastewater flowing into the treatment facility. This flow rate will be an important number for process control. Many wastewater treatment plants have a typical diurnal flow pattern as flow into the wastewater treatment plant is not constant. Over a 24 hour period, the flow can fluctuate significantly. There are usually two peaks during the day, typically in the late morning and evening. In the morning, the population that the treatment facility serves is waking up, taking showers, making coffee, cooking breakfast, and using the restroom. The flow tends to lessen in the late afternoon and then increases again in the evening. In the evening, a majority of the population is cooking dinner, washing dishes, doing laundry, and taking showers. In the middle of the night, most people are sleeping and not sending water down the drain.
There are several ways that flow can be measured. A simple way to measure flow is by using a weir. Weirs can either be square or V-notched, but the principle behind them is the same. As the flow of wastewater travels through a channel, the weir is placed in its path. The wastewater is then forced to move over the crest of the weir. Based on the dimension of the square or v-notched section in the weir, the flow can be determined by the height of the wastewater over the weir crest. If there is a significant amount of flow, then the height of wastewater will also be large. When the flow decreases, the height of wastewater over the weir will be lessened.
A Parshall Flume is another way to determine the flow rate entering or leaving the treatment facility. It works similarly to a weir in that the water is forced through a restriction in the channel and the height of water before and after the restriction is used to calculate a flow rate. All Parshall Flumes have a similar shape. There is a converging section, a throat section, and a diverging section. The converging section is where the flow is channeled and forced through a narrow section. The throat section is where the wastewater flow is restricted by traveling through a slimmer channel. This restriction will increase the velocity of the wastewater as it passes through the throat section and cause the level to rise in the converging section. Just like in a weir, higher flow rates will cause the wastewater level to increase and lower flow rates will show a decrease in the wastewater level. Many treatment plants have markings in the converging section to indicate the wastewater level. Modern treatment plants will have ultrasonic level transmitters that will record the level precisely. This data can be automatically transferred to a computer system where the flow rate is instantly calculated and recorded. A benefit of using the Parshall Flume instead of a weir is that the restriction in the channel will not cause any solids to settle out. In a weir, as water builds up behind it, the velocity is reduced and solids can settle out. Those solids will have to be removed periodically or odors can become problematic.
The most modern method of monitoring flow throughout a treatment facility is a magnetic flow meter, or “mag meter” for short. Mag meters work by having the wastewater flow through a pipe. The mag meter is connected to the pipe either by strapping the device to the outside of the pipe or more permanently, by bolting it between two sections of pipe. The device creates a magnetic field inside the pipe and as the water moves through, it creates a disturbance that is measured by the meter. Higher flow rates cause more of a disturbance. The meter can very accurately measure the flow of wastewater moving through the pipe. The benefit to mag meters is that there are no moving parts inside the pipe. So influent wastewater with lots of solids and organics will not interfere with the flow measurement. Mag meters are also very reliable and require little maintenance.
Some treatment facilities may have flow equalization. Recall that the incoming wastewater has an inconsistent flow rate that fluctuates drastically throughout a 24-hour period. This change in flow will also result in changing detention times which many of the treatment processes are dependent on. A treatment plant that utilizes flow equalization will attempt to even out the flow so it is consistent throughout the treatment plant all the time. By looking at historical data the plant operators will have a good idea of what the average flow rate for the plant is. This average flow rate is the target of the flow equalization. When the incoming wastewater is greater than the average flow, the excess amount will be directed towards a holding basin. When the flow is less than the average, typically in the middle of the night, the wastewater is pumped from the holding tank into the treatment facility. This can also reduce energy cost as day time pumping will be reduced. Energy costs are usually higher during peak periods. Some treatment facilities have flow equalization at the end of the treatment process. They may have restrictions in their discharge permit which may limit the flow rate leaving the facility. Other treatment plants don't have any flow equalization.
Sedimentation
After the incoming flow rate is measured the next step in the treatment process is primary sedimentation. The primary goal of sedimentation is to remove the settleable solids. A well-operated primary sedimentation tank can remove around 90% - 95% of settleable solids. There will also be a reduction in total suspended solids and a slight reduction in BOD5. Recall that settleable solids are the large solids in the wastewater and are measured by an Imhoff cone in the laboratory. The sedimentation process works because these solids are heavier, relative to the wastewater, and will, therefore, settle to the bottom of the tank. These tanks are referred to as Primary Clarifiers or Primary Sedimentation Basins. A clarifier is a tank or basin where the sedimentation process will occur. The rate at which the solids will settle is determined by Stokes Law, which takes into account the size of the solid particle, the specific gravity of the particle, and the specific gravity of the liquid. Specific gravity is a unitless number and is a measure of density relative to a reference liquid. For this book when talking about specific gravity, we will assume water is the reference liquid and it has a specific gravity of 1. When determining how quickly solids will settle in the sedimentation tank, the specific gravity of the solids will be a significant factor. If the solids have a specific gravity less than 1, they will not settle at all, in fact, they will float on the water. If the specific gravity is only slightly greater than one, the solids will settle but at a much slower rate than another particle with a larger specific gravity. This is fairly intuitive. Clearly the heavier the solids the quicker they will settle.
Another phenomenon that occurs in the sedimentation process is that as the solids collect at the bottom of the tank, the weight of the solids begin to compact and compress. This causes the solids to be thickened and have slightly less water content. Detention time, or how long the wastewater takes to travel through the tank, is a critical design parameter of primary sedimentation tanks. There needs to be enough time to allow the solids to settle but not so much time that the solids start to decompose. Decomposition will cause gas bubbles to form which can hinder solids settling and create foul odors.
Primary sedimentation tanks can either be circular or rectangular. Regardless of the configuration, the tanks will have similar components. At the inlet structure where the wastewater enters the tank, the velocity is typically high in order to prevent solids settling in the piping network as the wastewater comes from the preliminary unit process to the primary tank. Once in the primary tank, the velocity must be slowed. To accomplish this there will be some type of diffuser at the inlet end that will redirect flow and prevent short-circuiting. The dimensions of the primary sedimentation tank must be able to accommodate the flow of wastewater but must also reduce the velocity.
Flotation
As discussed earlier, the specific gravity of the material in the wastewater will determine how it interacts with the wastewater in the sedimentation tank. In addition to settling solids, primary tanks will also remove floatables. Typically, these floatables are classified as fats, oils, and grease (FOG). The specific gravity of these materials is less than 1 so they will rise to the top of the sedimentation tank and be removed. A rectangular primary tank will have flights that span the width of the tank. They are connected by a chain that is motor driven to slowly move with the flow of wastewater. The flights provide several functions. They prevent short-circuiting of material on the surface, they convey the FOG on the surface to a collection trough at the end of the tank, and they convey the settled solids to a hopper at the beginning of the tank. Circular tanks have a similar mechanism called a swing arm that provides the same functions.
Sludge Removal
Once the solids have settled to the bottom of the tank, they are conveyed to a hopper in the tank. Circular tanks are typically coned at the bottom so the solids build up in the center. Rectangular tanks have a hopper at the front of the tank and the flights convey the solids there. The solids must be removed from the tank periodically so they do not cause adverse conditions. There is organic matter in the solids and if it starts to decompose, it will create foul odors and gas bubbles that will hinder other solids from settling. Typically incoming wastewater is around 1% solids. Due to the sedimentation process, the percent solids concentration increases to around 4% to 8%. Due to the high amounts of solids, a standard pump cannot be used. Instead, special pumps including a progressive cavity type pump or a stator/rotor pump are commonly used.
The stator/rotor pump has a stationary part, the stator, and a rotating element, the rotor. Both have a corkscrew type shape and the rotor moves within the stator. As it rotates, it is passing the mixture of solids and water progressively through the pump. These pumps are designed to be able to handle the abrasive nature of the solids and not clog up. The solids are sent to an anaerobic digester where they are further treated and stabilized. Eventually, they will be dewatered and sent to a landfill for disposal.
Chemical Addition
To further enhance the sedimentation process, chemicals can be added. These chemicals will adhere to the solids and increase their specific gravity. With a higher specific gravity, the solids will settle more quickly. This can either be done to increase the amounts of solids and BOD5 or can be used to achieve average results in a smaller footprint. This process is referred to as chemically enhanced primary treatment (CEPT). Typically, ferrous or ferric chloride is used as the chemical aide. Other chemicals such as polymers may also be used. Polymers make the solids clump together which subsequently results in quicker settling in the sedimentation tank.
This process is not used as often anymore due to increasing regulatory requirements. Early in the wastewater industry, a treatment plant could have removed enough solids and BOD5 using the CEPT process. But as regulations became more stringent, many utilities upgraded to secondary treatment systems. This secondary biological treatment process is dependent on the BOD5. If too much is removed in the primary process, it will adversely affect treatment.


8.3: Second-Degree Price Discrimination
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Second-degree price discrimination can occur when there are different segments of consumers, for example, high demand customers and low demand customers. However, the firm is unable to accurately assign a customer to a segment prior to the sale. In this case, the seller provides volume discounts, essentially bundle pricing, but offers different amounts in the bundles at different prices. The separate bundles are carefully calibrated to extract the most surplus possible from each market segment. Schemes aimed at second-degree price discrimination are encountered every day (e.g., family-sized packaging).
The problem facing the seller is that he or she cannot assign consumers to one segment or another. Thus, the seller needs to set the bundles so that members of each segment self select into a particular bundle. To successfully implement second-degree price discrimination, the bundles must be designed so that the small bundle is most attractive to the low-demand customers and the large bundle is most attractive to the high-demand customers.
The firm must be careful when setting the volume discount. In fact, for second-degree price discrimination to work, the volume discount must satisfy the following conditions:
- The small bundle must be attractive to the low-demand segment (non-negative surplus).
- The large bundle must be attractive to the high-demand segment (non-negative surplus).
- The small bundle must provide a value to the low-demand segment that is at least as good as the large bundle.
- The large bundle must provide a value to the high-demand segment that is at least as good as the small bundle.
The first two conditions are called participation constraints. They will be satisfied provided the prices charged for the small and large bundles do not exceed the total willingness to pay of low-demand and high-demand customers, respectively. The last two conditions are called self-selection constraints and must be satisfied in order for the low-demand customers to voluntarily choose the small bundle and the high-demand customers to voluntarily choose the large bundle.
To meet these constraints, the firm should choose the size of the small bundle and set its price in order to extract the entire willingness to pay from the low-demand segment. By doing so, the firm gets as much as it can while still satisfying the participation constraint (specifically, the condition 1 above). Consumer surplus for low-demand customers will be zero.
The firm will then choose the size of the large bundle and set its price in order to capture as much of the high-demand customers’ willingness to pay as possible. However, unlike the low-demand segment, the firm will be unable to extract the entire magnitude of willingness to pay from the high-demand customers. Consequently, high-demand customers will have positive consumer surplus. To see why, note that the high-demand segment will derive a strictly positive net benefit from purchasing the small bundle. Consequently, if the firm were to set the large bundle and its price in a fashion that took all of the consumer surplus from high-demand customers, the high-demand customers would simply choose small bundles instead. In other words, the self-selection constraint (condition 4) would not hold. Thus, the firm must design and price the large bundle so that it provides at least as much surplus to high-demand customers as does the small bundle.
An example of a pricing scheme that meets all four conditions is presented in Figure \(\PageIndex{1}\). Panel A shows demand for a low-demand consumer. Panel B shows the demand for a high-demand consumer. The small bundle offered to the low-demand consumer consists of a two units. The low-demand consumer is charged his or her full willingness to pay (the area under the demand curve from 0 to 2 units) of $12. The size and price of the small bundle are determined in exactly the same way as described above under the bundle-pricing scheme.
The important thing is that a high-demand consumer gets positive surplus if they buy this small bundle. In this case, the high-demand consumer gets surplus of $4 from the small bundle. For this reason, the firm cannot charge the high-demand consumer his or her full willingness to pay for the large bundle. If it did, the high-demand consumer would simply buy small bundles in order to get positive surplus. Thus, the firm must set the price of the large bundle so that it provides at least $4 in surplus. The large bundle is set at three units. The large bundle price is set at no more than $17, which induces the high-demand consumer to purchase the large bundle. The per-unit price of the large bundle is $0.33 lower than the per unit price of the small bundle. Thus, the firm is giving the high-demand consumer a volume discount.
This volume discount is not offered because the firm is trying to be nice to its high-demand consumers. The volume discount allows it to make more money. It makes $4 by selling the small bundle. It makes $5 when it sells the large bundle. Thus, the ability to sell extra to the high-demand customers via the large bundle allows the firm to be more profitable. The volume discount described above is discriminatory because it allows the firm to price in a way that exploits differences in demand between the high- and low-demand segments. That said, not all volume discounts are discriminatory. For example, if it is less costly to package an deliver larger lots, volume discounts could represent differences in cost of providing the good in larger lots, not differences in willingness to pay.
Let us take a moment to verify that the example in Figure \(\PageIndex{1}\) meets the participation and self selection constraints.
- The small bundle must be attractive to the low-demand segment (non-negative surplus). This constraint is met in the example. The price of the small bundle just equals the willingness to pay of the low-demand segment. This is the highest the price can be and still have a bundle that is attractive enough for low-demand segment to participate. Surplus is zero but not negative.
- The large bundle must be attractive to the high-demand segment (non-negative surplus). This constraint is met in the above example. High-demand consumers get $4 of surplus from the large bundle.
- The small bundle must provide a value to the low-demand segment that is at least as good as the large bundle. In this example, the small bundle provides no consumer surplus to the low-demand consumer, but the large bundle provides -$4 of surplus.
- The large bundle must provide a value to the high-demand segment that is at least as good as the small bundle. In this example, it does; the large bundle provides surplus of $4 to the high-demand consumer. This is exactly what could have been obtained from the small bundle.


1.11: Lymphatic and Immune Systems
Learning Objectives
- Examine the anatomy of the lymphatic and immune systems
- Determine the main functions of lymphatic and immune systems
- Differentiate lymphatic and immune systems medical terms and common abbreviations
- Recognize the medical specialties associated with lymphatic and immune systems
- Discover common diseases, disorders, and procedures related to lymphatic and immune systems
Word Parts for the Lymphatic and Immune Systems
Click on prefixes, combining forms, and suffixes to reveal a list of word parts to memorize for the Lymphatic and Immune Systems.
Introduction to the Lymphatic and Immune Systems
The lymphatic system is a series of vessels, ducts, and trunks that remove interstitial fluid from the tissues and return it to the blood. The lymphatic vessels are also used to transport dietary lipids and cells of the immune system . Cells of the immune system, lymphocytes, all come from the hematopoietic system of the bone marrow. Primary lymphoid organs, the bone marrow and thymus gland, are the locations where lymphocytes proliferate and mature. Secondary lymphoid organs are the site in which mature lymphocytes congregate to mount immune responses. Many immune system cells use the lymphatic and circulatory systems for transport throughout the body to search for and then protect against pathogens.
This chapter begins by describing the anatomy and physiology of the lymphatic system, whose immune functions lead us into a discussion of the body’s multifaceted defenses, which together make up the immune system. Since the lymphatic system shares organs with a number of other body systems, the pathology discussed near the end of this chapter mainly focuses on disorders of the immune system.
Watch this video:
Media 11.1 Lymphatic System: Crash Course A&P #44 [Online video]. Copyright 2015 by CrashCourse .
Practice Medical Terms Related to the Lymphatic and Immune Systems
Anatomy and Physiology of the Lymphatic and Immune Systems
The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500 to 600 lymph nodes throughout the body (see Figure 11.1 ). Several organs and tissues that participate in immunity are also part of the lymphatic system.
Lymphatic Capillaries
An important function of the lymphatic system is to return the fluid (lymph) to the blood. Lymph may be thought of as recycled blood plasma. Blood pressure causes leakage of fluid from the blood capillaries, resulting in the accumulation of fluid in the interstitial space . In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary leakage. The blood vessels reabsorb 17 liters of this interstitial fluid , leaving three liters in the tissues for the lymphatic system to transport back to the circulation. If the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, interstitial fluid accumulates in the tissue spaces, causing a condition called lymphedema .
Lymphatic capillaries , also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body (see Figure 11.2 ). Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.
Did you know?
Lymphatic vessels and blood vessels are similar in structure and function. Lymph is not actively pumped by the heart but is forced through the vessels by the movements of the body muscles.
Larger Lymphatic Vessels, Trunks, and Ducts
The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 11.2 ).
In general, superficial lymphatics follow the same routes as veins, whereas deep lymphatic vessels of the viscera generally follow the paths of arteries. The superficial and deep lymphatics eventually merge to form larger lymphatic structures known as the lymphatic trunks . On the right side of the body, the right sides of the head, thorax, and right upper limb trunks drain lymph fluid into the right subclavian vein via the right lymphatic duct (see Figure 11.3). On the left side of the body, the trunks from the remaining portions of the body drain into the larger thoracic duct , which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli .
Primary Lymphoid Organs
The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body.
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Bone Marrow
- Recall that all blood cells, including lymphocytes, are formed in the red bone marrow. The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte , leaves the bone marrow and matures largely in the thymus gland .
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Thymus
- The thymus gland, where T cells mature, is a bilobed organ found in the space between the sternum and the aorta of the heart (see Figure 11.4 ). Connective tissue holds the lobes closely together but also separates them and forms a capsule .
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The loss of immune function with age is called
immunosenescence
. One major cause of age-related immune deficiencies is
thymic involution
.
- The shrinking of the thymus gland begins at birth at a rate of about 3% tissue loss per year. This shrinking continues until 35 to 45 years of age when the rate declines to about 1% loss per year for the rest of one’s life. At that pace, the total loss of thymic epithelial tissue and thymocytes would occur at about 120 years of age. So, in theory, 120 years could be the maximum life span.
Concept Check
- Do you remember what the suffix “-oid” means?
- Can you explain the term lymphoid ?
Secondary Lymphoid Organs
Lymphocytes develop and mature in the primary lymphoid organs , but they mount immune responses from the secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules . A naïve lymphocyte is one that has left the primary organ, where it learned to function immunologically, and entered a secondary lymphoid organ where it waits to encounter an antigen against which it will mount a response (see Figure 11.5 ).
Did you know?
The thymus gland produces a hormone called thymosin and is therefore also considered to be part of the endocrine system.
Lymph Nodes
Lymph nodes function to remove debris and pathogens from the lymph and are thus sometimes referred to as the “filters of the lymph” (see Figure 11.6 ). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system.
Spleen
The spleen is a vascular organ that is somewhat fragile due to the absence of a capsule. It is about 12 cm long and is attached to the lateral border of the stomach. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens.
Did you know?
You can live without your spleen. Do you remember the term for “surgical removal of the spleen”?
Lymphoid Nodules
The other lymphoid tissues, the lymphoid nodules , consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens.
Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens (see Figure 11.8 ). The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Tonsils have deep grooves called crypts, which accumulate all sorts of materials taken into the body through eating and breathing and actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are eliminated. A major function of tonsils is to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in children who have recurring throat infections since swollen palatine tonsils can interfere with breathing and/or swallowing.
Tonsils are named after their locations.
- Look at the figure above and determine which anatomical structure is closely associated with each set of tonsils and was therefore used to name the tonsils, for example, the lingual tonsils are named after the tongue (lingula).
- Can you tell which structures were used to name the palatine tonsils and the pharyngeal tonsils?
Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. These tissues, in addition to the tonsils, are effective against inhaled pathogens.
Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with mucous membrane. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances (see Figure 11.9 ). Peyer’s patches contain specialized cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted.
The Organization of the Immune System
The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways. The modern model of immune function is organized into a three-phase immune response (based on the timing of their effects). Ideally, this response will rid the body of a pathogen entirely (see Figure 11.10 ).
Think of a primary infection as a race between the pathogen and the immune system:
- The pathogen bypasses barrier defenses and starts to multiply in the host’s body.
- During the first 4 to 5 days, the innate immune response will partially control, but not stop the pathogen growth.
- The slower but more specific and effective adaptive immune response gears up and becomes progressively stronger, it will begin to clear the pathogen from the body. This clearance is referred to as seroconversion . It should be noted that seroconversion does not necessarily mean a patient is getting well.
Phase 1: Barrier Defenses
Barrier defenses are part of the body’s most basic innate defense mechanisms. They are not a response to infections, but rather are continuously working to protect against pathogens by preventing them from entering the body, destroying them after they enter, or flushing them out before they can establish themselves.

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Skin (sweat glands, sebaceous glands)
:
- Lower pH than pathogens prefer, may contain substances that are toxic to pathogens, washing action.
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Oral Cavity (salivary glands):
- Lysozyme is an enzyme that destroys bacteria.
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Stomach:
- Low pH which is fatal to many pathogens.
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Mucosal:
- Traps both microbes and debris, and facilitates their removal.
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Normal flora (nonpathogenic bacteria):
- Prevents pathogens from growing on mucosal surfaces.
Phase 2: Innate Immune Response
Innate immune responses are critical to the early control of infections. Whereas barrier defenses are the body’s first line of physical defense against pathogens, innate immune responses are the first line of physiological defense. Innate responses occur rapidly, but with less specificity and effectiveness than the adaptive immune response. Within the first few days of an infection, a series of antibacterial proteins are induced, each with activities against certain bacteria. Additionally, interferons are induced that protect cells from viruses in their vicinity. Finally, the innate immune response does not stop when the adaptive immune response is developed. In fact, both can cooperate and one can influence the other in their responses against pathogens.
Innate immune responses (and early induced responses) are in many cases ineffective at completely controlling pathogen growth but they slow pathogen growth and allow time for the adaptive immune response to strengthen and either control or eliminate the pathogen. The innate immune system also sends signals to the cells of the adaptive immune system, guiding them in how to attack the pathogen.
Watch this video:
Media 11.2 Immune System, Part 1: Crash Course A&P #45 [Online video]. Copyright 2015 by CrashCourse .
Cells of the Innate Immune Response
Phagocytes: Macrophages and Neutrophils
A phagocyte is a cell that is able to surround and engulf a particle or cell, a process called phagocytosis . The phagocytes of the immune system engulf other particles or cells, either to clean an area of debris, old cells, or to kill pathogenic organisms such as bacteria. Macrophages, neutrophils, and dendritic cells are the major phagocytes of the immune system and are the body’s fast acting, front line immunological defense against organisms that have breached barrier defenses and have entered the body.
Macrophages not only participate in innate immune responses but have also evolved to cooperate with lymphocytes as part of the adaptive immune response. Macrophages exist in many tissues of the body, either freely roaming through connective tissues or fixed to reticular fibers within specific tissues such as lymph nodes. When pathogens breach the body’s barrier defenses, macrophages are the first line of defense.
A neutrophil is a phagocytic cell that is attracted via chemotaxis from the bloodstream to infected tissues. contains cytoplasmic granules, which in turn contain a variety of vasoactive mediators such as histamine. Whereas macrophages act like sentries, always on guard against infection, neutrophils can be thought of as military reinforcements that are called into a battle to hasten the destruction of the enemy.
A monocyte is a circulating precursor cell that differentiates into either a macrophage or dendritic cell , which can be rapidly attracted to areas of infection by signal molecules of inflammation.
 If apoptosis is induced before the virus has the ability to synthesize and assemble all its components, no infectious virus will be released from the cell, thus preventing further infection.
Soluble Mediators of the Innate Immune Response
The previous discussions have alluded to chemical signals that can induce cells to change various physiological characteristics, such as the expression of a particular receptor. These soluble factors are secreted during innate or early induced responses, and later during adaptive immune responses.
Concept Check
Do you know the difference between these terms?
- Intercellular
- Intracellular
- Interstitial
Cytokines and Chemokines
A cytokine is a signaling molecule that allows cells to communicate with each other over short distances. Cytokines are secreted into the intercellular space, and the action of the cytokine induces the receiving cell to change its physiology. A chemokine is a soluble chemical mediator similar to cytokines except that its function is to attract cells (chemotaxis) from longer distances.
Early Induced Proteins
Early induced proteins are those that are not constitutively present in the body but are made as they are needed early during the innate immune response. Interferons are an example of early induced proteins. Cells infected with viruses secrete interferons that travel to adjacent cells and induce them to make antiviral proteins. Thus, even though the initial cell is sacrificed, the surrounding cells are protected.
Inflammatory Response
The hallmark of the innate immune response is inflammation . Stub a toe, cut a finger, or do any activity that causes tissue damage and inflammation will result with its four characteristics: heat, redness, pain, and swelling (“loss of function” is sometimes mentioned as a fifth characteristic). It is important to note that inflammation does not have to be initiated by an infection, but can also be caused by tissue injuries. The release of damaged cellular contents into the site of injury is enough to stimulate the response, even in the absence of breaks in physical barriers that would allow pathogens to enter (by hitting your thumb with a hammer, for example). The inflammatory reaction brings in phagocytic cells to the damaged area to clear cellular debris and encourages the entry of clotting factors to set the stage for wound repair. Inflammation also facilitates the transport of antigen to lymph nodes by dendritic cells for the development of the adaptive immune response.
The above image summarizes the following events in the inflammatory response:
- The released contents of injured cells stimulate the release of substances from mast cells including histamine, leukotrienes, and prostaglandins.
- Histamine increases blood flow to the area by vasodilation , resulting in heat and redness . Histamine also increases the permeability of local capillaries, causing plasma to leak out and form interstitial fluid, resulting in swelling.
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Leukotrienes
attract neutrophils from the blood by
chemotaxis
.
When local infections are severe, neutrophils are attracted to the sites of infections in large numbers, and as they phagocytose the pathogens and subsequently die, their accumulated cellular remains are visible as pus at the infection site. - Prostaglandins cause vasodilation by relaxing vascular smooth muscle and are a major cause of the pain associated with inflammation. Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen relieve pain by inhibiting prostaglandin production.
Concept Check
- Do you remember the suffix used to describe ‘inflammation’?
- Describe what causes the pain associated with inflammation.
Acute inflammation is a short-term innate immune response to an insult to the body. If the cause of the inflammation is not resolved, however, it can lead to chronic inflammation , which is associated with major tissue destruction and fibrosis.
Phase 3: Adaptive Immune Response
Watch this video:
Media 11.3 I mmune System, Part 2: Crash Course A&P #46 [Online video]. Copyright 2015 by CrashCourse .
Benefits of the Adaptive Immune Response
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Specificity
- The ability to specifically recognize and mount a response against almost any pathogen.
- Antigens are recognized by receptors on the surface of B and T lymphocytes.
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Immunological Memory
- The first exposure to a pathogen is called a primary adaptive response.
- Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective.
- Upon re-exposure to the same pathogen, a secondary adaptive immune response is generated, which is stronger and faster than the primary response, often eliminating the pathogen before it can cause damage or even symptoms.
- This secondary response is the basis of immunological memory , which gives us immunity .
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Self Recognition
- The ability to distinguish between self-antigens, those that are normally present in the body, and foreign antigens, those that might be on a potential pathogen.
- As T and B cells mature, there are mechanisms in place that prevent them from recognizing self-antigen, preventing a damaging immune response against the body. When these mechanisms fail, their breakdown leads to autoimmune diseases.
Lymphocytes: B Cells, T Cells, Plasma Cells
As stated above, lymphocytes are the primary cells of adaptive immune responses. These cells were introduced in the previous chapter and are summarized in the following table:
| CELL TYPE | DESCRIPTION AND DETAILS |
| Plasma Cell |
B cell (lymphocyte) that has been activated through exposure to an
antigen
and produces
antibodies
against that antigen (see the figure below).
There are 5 classes of antibodies (IgM, IgG, IgE, IgA, IgD), each functioning in different ways: IgM promotes chemotaxis, opsonization , and cell lysis, making it a very effective antibody against bacteria at early stages of a primary antibody response IgG is the one that crosses the placenta to protect the developing fetus from disease and exits the blood to the interstitial fluid to fight extracellular pathogens IgA is the only antibody to leave the interior of the body to protect body surfaces. IgA is also of importance to newborns, because this antibody is present in mother’s breast milk (colostrum), which serves to protect the infant IgE is associated with allergies and anaphylaxis |
| T Cell |
Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogen.
|
| Memory Cell |
B cells and T cells formed during primary exposure to a pathogen (see the figure below).
Remain in the body for a long time after infection and are able to mount a fast and effective immune response to a pathogen if it is encountered a second time, preventing the pathogen from causing disease. |
Active Versus Passive Immunity
Immunity to pathogens, and the ability to control pathogen growth so that damage to the tissues of the body is limited, can be acquired by:
- The active development of an immune response in the infected individual; or
- The passive transfer of immune components from an immune individual to a non-immune one.
The downside to this passive immunity is the lack of the development of immunological memory. Once the antibodies are transferred, they are effective for only a limited time before they degrade.
| IMMUNITY | NATURAL | ARTIFICIAL |
|---|---|---|
| Active: resistance to pathogens acquired during an adaptive immune response | Result of memory cells formed during the adaptive immune response to a pathogen | Vaccine response. Through vaccination, one avoids the disease that results from the first exposure to the pathogen, yet reaps the benefits of protection from immunological memory. Vaccination was one of the major medical advances of the twentieth century and led to the eradication of smallpox and the control of many infectious diseases, including polio, measles, and whooping cough |
| Passive: transfer of antibodies from an immune person to a nonimmune person | Trans-placental antibodies from mother to fetus and maternal antibodies in breast milk protect newborn from infections | Immunoglobulin injections taken from animals previously exposed to a specific pathogen; a fast-acting method of temporarily protecting an individual who was possibly exposed to a pathogen |
Evasion of the Immune System by Pathogens
The immune system and pathogens are in a slow, evolutionary race to see who stays on top. Early childhood is a time when the body develops much of its immunological memory that protects it from diseases in adulthood. Pathogens have shown the ability, however, to evade the body’s immune responses, as described below.
-
- Protective adaptations : It is important to keep in mind that although the immune system has evolved to be able to control many pathogens, pathogens themselves have evolved ways to evade the immune response. An example is in Mycobacterium tuberculosis , which has evolved a complex cell wall that is resistant to the digestive enzymes of the macrophages that ingest them, and thus persists in the host, causing the chronic disease tuberculosis.
-
- Multiple strains : Bacteria sometimes evade immune responses because they exist in multiple strains, each having different surface antigens and requiring individual adaptive immune responses. One example is a small group of strains of S. aureus , called methicillin-resistant Staphylococcus aureus (MRSA) , which has become resistant to multiple antibiotics.
-
- Antigen mutation: Because viruses’ surface molecules mutate continuously, viruses like influenza change enough each year that the flu vaccine for one year may not protect against the flu common to the next. New vaccine formulations must be derived for each flu season.
-
- Genetic recombination : An example is the influenza virus, which contains gene segments that can recombine when two different viruses infect the same cell. Recombination between human and pig influenza viruses led to the 2010 H1N1 swine flu outbreak.
-
- Immunosuppression : Pathogens, especially viruses, can produce immunosuppressive molecules that impair immune function.
Tissue Transplantation
With the use of tissue typing and anti-rejection drugs, transplantation of organs and the control of the anti-transplant immune response have made huge strides in the past 50 years. Immunosuppressive drugs such as cyclosporine A have made transplants more successful, but tissue matching is still key. Family members, since they share a similar genetic background, are much more likely to share major histocompatibility complex (MHC) molecules than unrelated individuals do.
One disease of transplantation occurs with bone marrow transplants, which are used to treat various diseases, including severe combined immunodeficiency disease (SCID) and leukemia. Because the bone marrow cells being transplanted contain lymphocytes capable of mounting an immune response, and because the recipient’s immune response has been destroyed before receiving the transplant, the donor cells may attack the recipient tissues, causing graft-versus-host disease. Signs and symptoms of this disease, which usually include a rash and damage to the liver and mucosa, are variable. Attempts have been made to moderate the disease by first removing mature T cells from the donor bone marrow before transplanting it.
Immune Responses Against Cancer
It is clear that with some cancers, like Kaposi’s sarcoma (see Figure 11.14 ), for example, that a healthy immune system does a good job at controlling them. This disease, which is caused by the human herpes virus, is almost never observed in individuals with strong immune systems. Other examples of cancers caused by viruses include liver cancer, caused by the hepatitis B virus, and cervical cancer, caused by the human papillomavirus. As these last two viruses have vaccines available for them, getting vaccinated can help prevent these two types of cancer by stimulating the immune response.
On the other hand, as cancer cells are often able to divide and mutate rapidly, they may escape the immune response, just as certain pathogens such as the human immunodeficiency virus (HIV) do.
There are three stages in the immune response to many cancers:
-
- Elimination occurs when the immune response first develops toward tumor-specific antigens specific to the cancer and actively kills most cancer cells.
- Equilibrium is the period that follows, during which the remaining cancer cells are held in check.
- Escape of the immune response, and resulting disease, occurs because many cancers mutate and no longer express any specific antigens for the immune system to respond to.
This fact has led to extensive research in trying to develop ways to enhance the early immune response to completely eliminate the early cancer and thus prevent a later escape. One method that has shown some success is the use of cancer vaccines. These differ from other vaccines in that they are directed against the cells of one’s own body. Treated cancer cells are injected into cancer patients to enhance their anti-cancer immune response and thereby prolong survival. The immune system has the capability to detect these cancer cells and proliferate faster than the cancer cells do, thus overwhelming the cancer in a similar way as they do for viruses. Cancer vaccines are being developed for malignant melanoma and renal (kidney) cell carcinoma.
Immune Responses and Stress
In order to protect the entire body from infection, the immune system is required to interact with other organ systems, sometimes in complex ways. For example, hormones such as cortisol (naturally produced by the adrenal cortex) and prednisone (synthetic) are well known for their abilities to suppress T cell immune mechanisms; hence, their prominent use in medicine as long-term, anti-inflammatory drugs.
One well-established interaction of the immune, nervous, and endocrine systems is the effect of stress on immune health. In the human vertebrate evolutionary past, stress was associated with the fight-or-flight response, largely mediated by the central nervous system and the adrenal medulla. This stress was necessary for survival since fighting or fleeing usually resolved the problem in one way or another. It has been found that short-term stress diverts the body’s resources towards enhancing innate immune responses. This has the ability to act fast and would seem to help the body prepare better for possible infections associated with the trauma that may result from a fight-or-flight exchange.
On the other hand, there are no physical actions to resolve most modern day stresses, including short-term stressors like taking examinations and long-term stressors such as being unemployed or losing a spouse. The effect of stress can be felt by nearly every organ system, and the immune system is no exception (see Table 11.3 ). Chronic stress, unlike short-term stress, may inhibit immune responses even in otherwise healthy adults. The suppression of both innate and adaptive immune responses is clearly associated with increases in some diseases.
| SYSTEM | STRESS-RELATED ILLNESS |
|---|---|
| Integumentary system | Acne, skin rashes, irritation |
| Nervous system | Headaches, depression, anxiety, irritability, loss of appetite, lack of motivation, reduced mental performance |
| Muscular and skeletal systems | Muscle and joint pain, neck and shoulder pain |
| Circulatory system | Increased heart rate, hypertension, increased probability of heart attacks |
| Digestive system | Indigestion, heartburn, stomach pain, nausea, diarrhea, constipation, weight gain or loss |
| Immune system | Depressed ability to fight infections |
| Male reproductive system | Lowered sperm production, impotence, reduced sexual desire |
| Female reproductive system | Irregular menstrual cycle, reduced sexual desire |
Anatomy Labeling Activity
Practice Terms Related to the Lymphatic and Immune Systems
Common Abbreviations for the Lymphatic and Immune Systems
Many terms and phrases related to the lymphatic and immune systems are abbreviated. Learn these common abbreviations by expanding the list below.
Diseases and Disorders of the Lymphatic and Immune Systems
The immune response can be under-reactive or over-reactive, leading to a state of disease. The factors that maintain immunological homeostasis are complex and incompletely understood.
Lymphedema
Lymphedema is a condition in which lymphatic fluid builds up in the body’s soft tissues. Lymphedema often occurs in the arms and legs, although other body parts may be affected. There are two types of lymphedema: primary and secondary. Primary lymphedema may be congenital and is caused by abnormalities in the lymph system. Secondary lymphedema occurs when the lymph system is damaged or blocked due to injury, cancer, or cancer treatment. Signs and symptoms include swelling of the limbs and digits , difficulty moving the joints in the arms and legs, and the sensation that the skin is too tight. Lymphedema is rarely cured. Instead, treatment such as the use of pressure garments, light exercise, massage therapy, and surgery can reduce symptoms and lower the risk of complications. Patients with severe cases of lymphedema are at an increased risk of developing lymphangiosarcoma, an aggressive cancer of the lymph vessels (National Cancer Institute, 2021; Sleigh & Manna, 2021). For more information, visit the Mayo Clinic’s web page on lymphedema .
Underactive Immune System: Immunodeficiencies
Suppressed immunity can result from inherited genetic defects or by acquiring viruses.
Inherited Immunodeficiencies/SCID
While many inherited immunodeficiencies exist, the most serious is severe combined immunodeficiency disease (SCID). This complex disease is caused by many different genetic defects which result in impaired B cell and T cell arms of the adaptive immune response. Children with this disease usually die of opportunistic infections within their first year of life unless they receive a bone marrow transplant. Such a procedure had not yet been perfected for David Vetter, the “boy in the bubble,” who was treated for SCID by having to live in a sterile plastic cocoon for the 12 years before his death from infection in 1984. One of the features that make bone marrow transplants work as well as they do is the proliferative capability of hematopoietic stem cells of the bone marrow. Only a small amount of bone marrow from a healthy donor is given intravenously to the recipient. It finds its own way to the bone where it populates it, eventually reconstituting the patient’s immune system, which is usually destroyed beforehand by treatment with radiation or chemotherapeutic drugs.
New treatments for SCID using gene therapy, inserting non-defective genes into cells taken from the patient and giving them back, have the advantage of not needing the tissue match required for standard transplants. Although not a standard treatment, this approach holds promise, especially for those in whom standard bone marrow transplantation has failed.
Acquired Immunodeficiency/HIV and AIDS
Although many viruses cause suppression of the immune system, only HIV wipes it out completely. HIV is transmitted through semen, vaginal fluids, and blood, and can be caught by risky sexual behaviors and the sharing of needles by intravenous drug users. There are sometimes, but not always, flu-like symptoms in the first 1 to 2 weeks after infection. The presence of anti-HIV antibodies indicates a positive HIV test. Because seroconversion takes different lengths of time in different individuals, multiple HIV tests are given months apart to confirm or eliminate the possibility of infection.
After seroconversion, the amount of virus circulating in the blood drops and stays at a low level for several years. During this time, the levels of CD4 T cells decline steadily, until at some point, the immune response is so weak that opportunistic disease and eventually death result.
Treatment for the disease consists of drugs that target virally encoded proteins that are necessary for viral replication but are absent from normal human cells. By targeting the virus itself and sparing the cells, this approach has been successful in significantly prolonging the lives of HIV-positive individuals.
Overactive Immune System: Hypersensitivities and Autoimmune Diseases
Hypersensitivities
Over-reactive immune responses include the hypersensitivities: allergies and inflammatory responses to nonpathogenic environmental substances. The table below compares different hypersensitivities.
| TYPE OF HYPERSENSITIVITY | DETAILS AND EXPLANATION |
| Type I |
|
| Type II |
|
| Type III |
|
| Type IV |
|
Autoimmune Responses
The worst cases of the immune system overreacting are autoimmune diseases in which the immune systems begin to attack cells of the patient’s own body, causing chronic inflammation and significant damage. The trigger for these diseases is often unknown, although environmental and genetic factors are likely involved. Treatments are usually based on resolving the symptoms using immunosuppressive and anti-inflammatory drugs. Figure 11.15 below provides two examples of autoimmune diseases: rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE).
Overall, there are more than 80 different autoimmune diseases, which are a significant health problem in the elderly. Table 11.5 below lists several of the most common autoimmune diseases, the antigens that are targeted (autoantigen or “self” antigen), and the resulting tissue damage.
| DISEASE | AUTOANTIGEN | SYMPTOMS |
|---|---|---|
| Celiac disease | Tissue transglutaminase | Damage to small intestine |
| Diabetes mellitus type I | Beta cells of pancreas | Low insulin production; inability to regulate serum glucose |
| Graves’ disease | Thyroid-stimulating hormone receptor (antibody blocks receptor) | Hyperthyroidism |
| Hashimoto’s thyroiditis | Thyroid-stimulating hormone receptor (antibody mimics hormone and stimulates receptor) | Hypothyroidism |
| Lupus erythematosus | Nuclear DNA and proteins | Damage of many body systems |
| Myasthenia gravis | Acetylcholine receptor in neuromuscular junctions | Debilitating muscle weakness |
| Rheumatoid arthritis | Joint capsule antigens | Chronic inflammation of joints |
Lymphoma
Lymphoma is a form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues. As in leukemia, the malignant leukocytes do not function properly, and the patient is vulnerable to infection. Some forms of lymphoma tend to progress slowly and respond well to treatment. Others tend to progress quickly and require aggressive treatment, without which they are rapidly fatal.
Medical Terms in Context
Medical Specialties and Procedures Related to the Lymphatic and Immune Systems
Clinical immunologists and allergists diagnose and treat diseases of the immune system (National Center for O*Net Development, 2021). For more information, please visit the American Academy of Allergy, Asthma, and Immunology’s web page on careers in immunology .
Skin Testing
Skin testing (for allergies) is done by a clinical immunologist/allergist to identify allergens in Type I hypersensitivity. In skin testing, allergen extracts are injected into the epidermis, and a positive result of the wheal and flare response usually occurs within 30 minutes. The soft center is due to fluid leaking from the blood vessels and the redness is caused by the increased blood flow to the area that results from the dilation of local blood vessels at the site.
Lymphatic and Immune Systems Vocabulary
Active immunity : Immunity developed from an individual’s own immune system.
Acute inflammation : Inflammation occurring for a limited time period; rapidly developing.
Adaptive immune response : A relatively slow but very specific and effective immune response controlled by lymphocytes.
Afferent lymphatic vessels : Vessels that lead into a lymph node.
Allergens : Antigens that evoke type 1 hypersensitivity (allergy) responses.
Allergist : Specialist who specializes in treating individuals with a hypersensitivity to allergens.
Allergy : Inflammatory response due to a hypersensitivity to a substance that normally is harmless or would not cause an immune response in most people.
Anaphylactic shock : A severe and sometimes life-threatening immune system reaction to an antigen that a person has been previously exposed to. The reaction may include itchy skin, edema, collapsed blood vessels, fainting, difficulty in breathing, and death.
Antibody: A protein made by plasma cells (a type of white blood cell) in response to an antigen (a substance that causes the body to make a specific immune response). Each antibody can bind to only one specific antigen. The purpose of this binding is to help destroy the antigen.
Antigens : Substances that provokes an immune response. This happens because the immune system sees the antigen as foreign, or ‘non-self” (does not belong in that body).
Apoptosis : Programmed cell death.
Autoimmune diseases/disorders : Disorders in which the immune system overreacts and begins to attack itself.
B cells : Lymphocytes that act by differentiating into an antibody-secreting plasma cell.
Barrier defenses: Antipathogen defenses deriving from a barrier that physically prevents pathogens from entering the body to establish an infection.
Bone marrow : Tissue found inside bones; the site of all blood cell differentiation and maturation of B lymphocytes.
Bronchus-associated lymphoid tissue (BALT) : Lymphoid nodule associated with the respiratory tract.
CD4 T Cells: CD4 is the receptor that HIV uses to get inside T cells and reproduce. CD4+ helper T cells play an important role in T cell immune responses and antibody responses.
Chemokine : Soluble, long-range, cell-to-cell communication molecule.
Chemotaxis : Movement in response to chemicals; a phenomenon in which injured or infected cells and nearby leukocytes emit the equivalent of a chemical “911” call, attracting more leukocytes to the site.
Chronic inflammation : Ongoing inflammation that can be caused by foreign bodies, persistent pathogens, and autoimmune diseases such as rheumatoid arthritis.
Chyle : Lipid-rich lymph inside the lymphatic capillaries of the small intestine.
Cisterna chyli : A sac-like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs by way of the left and right lumbar trunks and the intestinal trunk.
Complement : Enzymatic cascade of constitutive blood proteins that have antipathogen effects, including the direct killing of bacteria.
Cytokine : A signaling molecule that allows cells to communicate with each other over short distances.
Deep lymphatic vessels: Lymphatic vessels of the organs.
Efferent lymphatic vessels : Vessels that lead out of a lymph node.
Erythroblastosis fetalis : An immune reaction between maternal and fetal blood due to the Rh antigen; also known as hemolytic disease of the newborn (HDN).
Genetic recombination: The combining of gene segments from two different pathogens.
Graft-versus-host disease (GVHD) : A condition that can occur in bone marrow transplant recipients; occurs when the transplanted cells mount an immune response against the recipient’s tissue.
Histamine : A vasodilator involved in the inflammatory response.
Human immunodeficiency virus (HIV) : An infectious disease transmitted through semen, vaginal fluids, and blood that suppresses the immune system. HIV infection may be managed with antiviral drugs or may progress to acquired immune deficiency syndrome (AIDS).
Hypersensitivities : Reacting to something that would not normally evoke a reaction.
Immune system : Series of barriers, cells, and soluble mediators that combine to response to infections of the body with pathogenic organisms.
Immunity: Resistant to the effects of pathogens.
Immunodeficiency : The decreased ability of the body to fight infections and other diseases.
Immunological memory : Ability of the adaptive immune response to mount a stronger and faster immune response upon re-exposure to a pathogen.
Immunology : The study of the body’s immune system.
Induration : A firm, raised reddened patch of skin.
Inflammation : Basic innate immune response characterized by heat, redness, pain, and swelling.
Innate immune response : Fast-acting non-specific immune mechanisms that are present from birth.
Intercellular : Between cells.
Interferons : Early induced proteins made in virally infected cells that cause nearby cells to make antiviral proteins.
Interstitial fluid : Extracellular fluid not contained within blood vessels.
Interstitial space: Spaces between individual cells in the tissues.
Intracellular : Inside the cell membrane or within the cell.
Leukemia: A cancer involving an abundance of leukocytes.
Lymph : The term used to describe interstitial fluid once it has entered the lymphatic system.
Lymph node : One of the bean-shaped organs found associated with the lymphatic vessels.
Lymphatic capillaries : Smallest of the lymphatic vessels and the origin of lymph flow.
Lymphatic system: Network of lymphatic vessels, lymph nodes, and ducts that carries lymph from the tissues and back to the bloodstream.
Lymphatic trunks : Large lymphatics that collect lymph from smaller lymphatic vessels and empties into the blood via lymphatic ducts.
Lymphocytes : The second most common type of leukocyte and are essential for the immune response.
Lymphoid nodules: Unencapsulated patches of lymphoid tissue found throughout the body.
Lymphoma : A form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues. These leukocytes do not function properly, and the patient is vulnerable to infection.
Macrophage : A large cell derived from a monocyte; they participate in innate immune responses.
Major histocompatibility complex (MHC): Protein structures found on the outside of cells that help the immune system recognize non-self antigens.
Mast cell : Cell found in the skin and the lining of body cells that contains cytoplasmic granules with vasoactive mediators such as histamine.
Memory T cells : Long-lived immune cells reserved for future exposure to a pathogen.
Monocyte : A type of immune cell that is made in the bone marrow.
Mucosa-associated lymphoid tissue (MALT) : Lymphoid nodule associated with the mucosa.
Mucous membranes : Epithelial membranes that line the body cavities and hollow passageways that open to the external environment.
Naïve lymphocyte : Mature B or T cell that has not yet encountered antigen for the first time.
Natural killer cell (NK) : Cytotoxic lymphocyte of innate immune response.
Neutrophil : Phagocytic white blood cell recruited from the bloodstream to the site of infection via the bloodstream.
Opsonization : A process by which an antibody or an antimicrobial protein binds to a pathogen, thereby marking it as a target for phagocytes.
Passive immunity : Transfer of immunity to a pathogen to an individual that lacks immunity to this pathogen usually by the injection of antibodies.
Pathogen : An organism that causes a disease.
Phagocytosis : Movement of material from the outside to the inside of the cells via vesicles made from invaginations of the plasma membrane; process where some white blood cells engulf invading microorganisms.
Plasma cells : A type of B lymphocyte that produces antibodies, which bind to specific foreign or abnormal antigens in order to destroy them.
Primary adaptive response : Immune system’s response to the first exposure to a pathogen.
Primary lymphoid organs : Site where lymphocytes mature and proliferate; for example, red bone marrow and the thymus gland.
Right lymphatic duct : Drains lymph fluid from the upper right side of the body into the right subclavian vein.
Staphylococcus aureus : A bacteria that is commonly found in minor skin infections, as well as in the nose of some healthy people.
Secondary adaptive response : Immune response observed upon re-exposure to a pathogen, which is stronger and faster than a primary response.
Secondary lymphoid organs : Sites where lymphocytes mount adaptive immune responses, examples include lymph nodes and spleen.
Seroconversion: The reciprocal relationship between virus levels in the blood and antibody levels.
Severe combined immunodeficiency disease (SCID) : A rare, inherited disease that is marked by a lack of B and T lymphocytes.
Spleen : Secondary lymphoid organ that filters pathogens from the blood (white pulp) and removes degenerating or damaged blood cells (red pulp).
Superficial lymphatics : Lymphatic vessels of the subcutaneous tissues of the skin.
Systemic lupus erythematosus (SLE) : A chronic, inflammatory, connective tissue disease that can affect the joints and many organs; also called lupus.
T cell : Lymphocyte that acts by secreting molecules that regulate the immune system or by causing the destruction of foreign cells, viruses, and cancer cells.
Thoracic duct : Large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head.
Thymocytes : A type of white blood cell that is part of the immune system and develops from stem cells in the bone marrow; also called T cells and T lymphocytes.
Thymus : Primary lymphoid organ, where t lymphocytes proliferate and mature.
Tissue typing: The determination of major histocompatibility complex (MHC) molecules in the tissue to be transplanted to better match the donor to the recipient.
Tonsils : Lymphoid nodules associated with the nasopharynx.
Vaccine : A killed or weakened pathogen or its components that, when administered to a healthy individual, leads to the development of immunological memory (a weakened primary immune response) without causing much in the way of symptoms.
Vasodilation : The physiological widening of blood vessels by relaxing the vascular smooth muscle.
Wheal and flare response : A soft, pale swelling at the site surrounded by a red zone.
Test Yourself
References
CrashCourse. (2015, November 30). Lymphatic system: Crash course A&P #44 [Video]. YouTube. https://youtu.be/I7orwMgTQ5I
CrashCourse. (2015, December 8). Immune system, part 1: Crash course A&P #45 [Video]. YouTube. https://youtu.be/GIJK3dwCWCw
CrashCourse. (2015, December 14). I mmune system, part 2: Crash course A&P #46 [Video]. YouTube. https://youtu.be/2DFN4IBZ3rI
National Center for O*NET Development. (2021) 29-1229.01 – Allergists and Immunologists. O*NET OnLine . Retrieved November 2, 2021, from https://www.onetonline.org/link/summary/29-1229.01
National Cancer Institute. (2021). Lymphedema (PDQ®)–Patient version . National Institutes of Health. https://www.cancer.gov/about-cancer/treatment/side-effects/lymphedema/lymphedema-pdq
Sleigh, B. C., & Manna, Bi. (2021). Lymphedema. In StatPearls [Internet] . https://www.ncbi.nlm.nih.gov/books/NBK537239/
Image Descriptions
Figure 11.1 image description: The left panel shows a female human body, and the entire lymphatic system is shown Labels read (clockwise from top): thymus, lymph nodes, thymus, spleen, lymph vessel, bone marrow, right lymphatic duct, entering vein, tonsil, adenoid. The right panel shows magnified images of the thymus and the lymph node. Labels read (clockwise from top): tissue cell, interstitial fluid, lymphatic capillary, blood capillary, lymphatic vessel. Label of lymph node reads masses of lymphocytes and macrophages. [Return to Figure 11.1].
Figure 11.2 image description: This image shows the lymph capillaries in the tissue spaces. Labels read (clockwise, from top): lymph capillary, tissue cells, venule, lymphatic vessel, tissue fluid, arteriole). It also shows a magnified image showing the interstitial fluid and the lymph vessels. Labels read (clockwise, from top): collagen fiber, interstitial fluid, lymph, lymph vessel endothelial cells, backflow prevention valve, endothelial flaps. [Return to Figure 11.2].
Figure 11.3 image description: This figure shows the lymphatic trunks and the duct system in the human body. Labels read (clockwise from top) thoracic duct, cisterna chyli of thoracic duct, drained by thoracic duct, drained by right lymphatic duct. Callouts to the left and right show the magnified views of the left and right jugular vein respectively. Labels read (right lymphatic duct): right internal jugular vein, right subclavian vein, right lymphatic duct; (left jugular vein): left internal jugular vein, thoracic duct drains into subclavian vein, left subclavian vein. [Return to Figure 11.3].
Figure 11.4 image description: The left panel of this figure shows the head and chest of a woman and the location of the thymus is marked. Labels read (clockwise, from top) lymph nodes, spleen, heart, thymus, right lymphatic duct entering vein, tonsil, adenoid. The top right panel shows a micrograph of the thymus. Labels read (from left to right): medulla, cortex, trabeculae, fibrous capsule. The bottom right panel shows a magnified view of the structure of the thymus. Labels read (clockwise, from top): thymocytes, trabecula, fibrous capsule, cortex, medulla (layers), medullary epithelial cell, blood vessel, macrophage, dendritic cell, cortical epithelial cell. [Return to Figure 11.4].
Figure 11.5 image description: This flowchart shows the process in which a naïve T cell becomes activated T cells in the left part of the pathway and memory cells in the right part of the pathway. A naive T cell becomes an activated T cell when an antigen-presenting cell is introduced. The antigen is extracted from a pathogen and then either activated T cells are cloned and destroy the infected cells in the body, and/or memory T cells are produced and are activated if this antigen is encountered again. [Return to Figure 11.5].
Figure 11.6 image description: The left panel of this figure shows a micrograph of the cross section of a lymph node. Labels indicate the connective tissue capsule, cortex, and subcapsular sinus. The right panel shows the structure of a lymph node. Labels indicate (from top, clockwise) the efferent lymphatic vessels, connective tissue capsule, subcapsular sinus, cortex, afferent lymphatic vessels, trabecula, and germinal centers. [Return to Figure 11.6].
Figure 11.7 image description: The top left panel shows the location of the spleen in the human body. The top center panel shows a close up view of the location of the spleen. Labels read (clockwise, from top): hilum, spleen, diaphragm, splenic vein, splenic artery. The top right panel shows the blood vessels and spleen tissue. Labels read (from left to right, top then bottom) red pulp, trabecula (bottom) white pulp, arteriole, venule. The bottom panel shows a histological micrograph Labels read (clockwise, from top): trabecula, marginal zone, central artery or arteriole, germinal center, venous sinus, red pulp, arterial capillaries. [Return to Figure 11.7].
Figure 11.8 image description: The top panel of this image shows the locations of the tonsils. Labels read (clockwise from top):palatine tonsil, palatine bone, tongue, mandible, hyoid, trachea, esophagus. Callout shows the location of the pharyngeal tonsil. Labels read (from top): brain, sphenoidal sinus, sphenoid bone, pharyngeal tonsil, nasopharynx. Another callout details the location of the palatine tonsil. Labels read (from top): palatine tonsil, lingual tonsil, epiglottis. Another callout shows a photograph of the back of the throat where the tonsils are located. Labels read (from top) hard palate, soft palate, uvula, palatine tonsils (swollen due to infection) and tongue. The bottom panel shows the histological micrograph of the tonsils. Labels read (from top): crypt, stratified squamous epithelium, germinal centers. [Return to Figure 11.8].
Figure 11.9 image description: This figure shows a micrograph of a mucosa associated lymphoid tissue (MAST) nodule. Labels indicate the mucosa and Peyer’s patches (which appear to be dark purple). [Return to Figure 11.9] .
Figure 11.10 image description: This figure shows a lateral view of a human face in the top left. A magnified callout shows the germinal center of the palatine tonsil. Another magnified view shows how the innate immune system works. This process is described in greater detail in the text below the figure. [Return to Figure 11.10] .
Figure 11.11 image description: The top panel of this figure shows the mast cells detecting an injury and initiating an inflammatory response. The bottom panel shows the increase in blood flow in response to histamine. [Return to Figure 11.11] .
Figure 11.12 image description: This graph shows the antibody concentration as a function of time in primary and secondary response. Initial exposure indicates a low concentration of antibody, which then elevates over time during the primary immune response. It decreases a little during secondary exposure, but then spikes during the secondary immune response. [Return to Figure 11.12].
Figure 11.13 image description: This flowchart shows how the clonal selection of B cells takes place. The left panel shows the primary response and the right panel shows the secondary response. During a primary B cell immune response, both antibody-secreting plasma cells and memory B cells are produced. These memory cells lead to the differentiation of more plasma cells and memory B cells during secondary responses. [Return to Figure 11.13].
Figure 11.14 image description: This image shows Kaposi’s Sarcoma lesions on the surface of the skin. [Return to Figure 11.14].
Figure 11.15 image description: The left panel of this figure shows an x-ray image of a person’s hand with rheumatoid arthritis, and the right panel of this figure shows a woman’s body with labels showing the different responses in the body when the patient suffers from lupus.  [Return to Figure 11.15].
Unless otherwise indicated, this chapter contains material adapted from Anatomy and Physiology (on OpenStax ), by Betts et al. and is used under a CC BY 4.0 international license . Download and access this book for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction


1.33: Staying on track- reducing missed immunization opportunities in the pediatric population
Introduction
Current immunization recommendations are published by the Centers for Disease Control and Prevention (CDC) based on recommendations from Advisory Committee on Immunization Practices (ACIP). Recommendations are based on several criteria including age, risk factors, and chronic diseases. 1 Pharmacists must be familiar with the immunization schedules as well as strategies for catching up on missed doses. The CDC’s Recommended Child and Adolescent Immunization Schedule for ages 18 years or younger identifies the vaccines routinely recommended by age and when catch-up immunizations are considered appropriate, should a dose be delayed. The catch-up schedule provides users with vaccine specific guidance on minimum intervals between doses, maximum ages for administration, and conditions under which additional doses may not be necessary. 2 The American Academy of Pediatrics (AAP) also publishes schedules and the CDC publishes best practices guidelines. 3,4
Despite the many opportunities to receive immunizations at primary care offices, pharmacies, health departments, and employer programs that exist, overall immunization rates remain below Healthy People 2030 goals for most communities. 5 Although pharmacists are trusted resources for provision of immunizations in adults, they have not traditionally had a large role in providing pediatric immunizations, with most given in a pediatrician’s office. Across various pharmacy practice settings, a pharmacist could recognize a child as not up-to-date on immunizations, but due to scope of practice limitations, children either went unimmunized or relied on pharmacist referral, which constitute missed opportunities. Additionally, the decrease in childhood wellness visits as a result of the COVID-19 pandemic placed constraints on the usual opportunities for immunizations for pediatric patients. 6
Before the COVID-19 pandemic, some states had already expanded pharmacist practice for pediatric immunizations as a solution to care access issues. In response to declining rates of pediatric immunizations and to reduce barriers to immunizations, the US Department of Health and Human Services issued a directive on August 19, 2020 (the Third Amendment to the declaration under the Public Readiness and Emergency Preparedness Act) to allow all pharmacists and supervised qualified pharmacy interns and technicians to administer any CDC approved immunization to individuals three to 18 years of age without a collaborative practice agreement or protocol. 7 This effectively expanded ability for pharmacists in all 50 states to immunize pediatric patients. The amendment was subject to various requirements, including:
- Only using FDA-approved or emergency use authorized vaccines
- Ordering/administration according to CDC ACIP immunization schedules
- Compliance with jurisdictional recordkeeping and reporting requirements
- Informing patients/caregivers the importance of a well-child visit with a pediatrician
- Completion of at least 20 hours of practical training, current cardiopulmonary resuscitation certification and (for licensed pharmacists) a minimum of two hours of Accreditation Council for Pharmacy Education approved, immunization-related continuing pharmacy education during each state licensing period
Delays in immunizations can be tied to individual factors such as cost or vaccine hesitancy; however, others are more structural, including access to care, vaccine distribution, misinformation, or more recently, the COVID-19 global pandemic. With patients or their caregivers, it is important to have discussions on why immunizations are not up-to-date to identify and reconcile any barriers as needed. From a public health perspective, increasing the access to care, such as allowing pharmacists to immunize children, or marketing campaigns directed at vaccine education, can help to fight barriers at a population level. Providing clinics at local schools, libraries, community centers, or other organizations that are centrally located and providing vaccines or education may be an effective strategy to reduce access barriers. 7,8
Case
You are an ambulatory care pharmacist practicing in a family medicine clinic in a medically underserved area, serving patients in both your local community and surrounding rural areas.
CC: “My grandson hasn’t been able to get into his pediatrician in a while and I’m concerned that he is falling behind on his shots.”
Patient: AP is a 15-month-old (32 in, 10.9 kg) white male who is brought in to establish care as a new patient. His grandmother is his legal guardian and accompanies him today.
HPI: unremarkable
PMH: unremarkable
FH:
- Mother OUD
- Father (deceased) suicide; AUD
- Maternal grandmother T2DM
SH: Second-hand tobacco exposure (grandfather smokes 10 cigarettes per day)
Medications: None
Allergies: NKDA
Labs: unremarkable
ROS: unremarkable
SDH: AP lives with his grandparents (both in their 50s) and his four-year-old sister. He has medical and prescription coverage through the state Medicaid program. His grandfather works full-time for a utility company and his grandmother recently quit her retail job to care for the grandchildren. AP’s mother has weekly supervised custody visits.
Additional context: AP’s grandmother reports that while AP was scheduled for a 12-month-old well child visit with his previous provider, this appointment was canceled due to COVID-19. She does not have his immunization records but reports his last vaccines were received prior to her gaining custody two months ago. She is concerned about the costs for this visit as the family is on a fixed income and has spent much of their savings on legal fees related to custody of their grandchildren.
Case Questions
- What are the long-term implications of missed childhood immunizations upon a community?
- What role could the pharmacist in AP’s primary care setting play in bringing his immunizations up to date? What role might a pharmacist have in bringing immunizations up to date in the following practice settings: (1) community; (2) acute care; or (3) managed care?
- What tools are available to healthcare professionals to determine a patient’s immunization record when the patient or caregiver is not able to provide one?
- You manage to track down the following records from AP’s previous pediatric provider. Using the CDC Pediatric Immunization catch-up schedule, which vaccines would you recommend for AP’s visit today?
| Vaccine | Age | |||
|---|---|---|---|---|
| 1 Day | 2 months | 5 months | 10 months | |
| HepB | Dose #1 | Dose #2 | Dose #3 | Dose #4 |
| RV* | Dose #1 | Dose #2 | ||
| DTaP | Dose #1 | Dose #2 | Dose #3 | |
| Hib* | Dose #1 | Dose #2 | Dose #3 | |
| PCV13 | Dose #1 | Dose #2 | Dose #3 | |
| IPV | Dose #1 | Dose #2 | Dose #3 | |
| IIV/LAIV | ||||
| MMR | ||||
| VAR | ||||
| HepA | ||||
| *documentation is missing which product was given |
- AP requires multiple vaccines at today’s visit. Explain how you will ensure that he will be brought up to date while avoiding unnecessary clinic visits.
- Referring to the CDC’s Pink Book, how will you address each of the following special considerations for pediatric immunizations specific to AP: (1) education; (2) site selection; (3) needle length/gauge; and (4) comfort measures and after-care?
- What strategies would you suggest to address the cost barriers identified by AP’s grandmother?
- What strategies could you consider if the following barriers were present: (1) safety/vaccine hesitancy; (2) transportation to the physician’s office; or (3) lack of a PCP?
- How would you ensure the continuity of care between you and the other members of AP’s healthcare team?
Author Commentary
Pharmacists are respected immunizers. Because many children missed vital doses in their recommended immunization schedules due to the COVID-19 pandemic, pharmacists were given expanded ability to immunize in the pediatric population age three and over in all 50 states. Therefore, it is the responsibility of the pharmacist or student pharmacist to stay current and to develop competency in pediatric immunizations through additional training or completing a continuing education program on pediatric immunizations. 9
As regulations on obtaining childhood immunizations have loosened, patients require assistance to understand the importance of and pathway to catching up on missed immunizations. Oftentimes, patients present as part of a family to the pharmacy, and this is an opportunity to assess patients for missed doses. Depending on state regulations, pharmacists can also consider becoming a Vaccine For Children (VFC) program provider, which enables delivery of no-cost vaccines to children who are uninsured, under insured, Medicaid-eligible or American Indian or Alaska Native. 10 Regardless of practice settings, pharmacists should be an active participant of the “immunization neighborhood” by sharing immunization records, making referrals, and providing information and professional advice to raise immunization rates. The COVID-19 pandemic and associated issues it has created an all-hands-on deck approach to care and children should be included in the pharmacist’s approach to immunization completion where possible. 9,11
Patient Approaches and Opportunities
It is widely known that pharmacists are the most accessible healthcare professionals. With expanding pharmacist roles in pediatric immunizations, barriers such as vaccine hesitancy, transportation to the pediatrician, and access issues can be overcome. The CDC Recommended Child and Adolescent Immunization Schedule for ages 18 years or younger and the catch-up schedule are key resources in determining what immunizations a child requires and how to bring delayed immunizations up-to-date. The pharmacist ensures continuity of care by recording vaccinations administered in the pharmacy in the pharmacy’s immunization system, and the state registries or Immunization Information Systems (IIS), and by communicating this information with the patient’s healthcare team. Additionally the pharmacist should inform parents/caregivers on the importance of a well-child visit with their pediatrician or family medicine provider to assess growth and developmental milestones to ensure the optimal physical, mental, and social health of the child.
Related chapters of interest:
- Communicating health information: hidden barriers and practical approaches
- Deciphering immunization codes: making evidence-based recommendations
- Getting to the point: importance of immunizations for public health
- Laying the foundation for public health priorities: Healthy People 2030
External resources:
- Immunization Action Coalition. Clinic tools. https://www.immunize.org/clinic/scheduling-vaccines.asp
- Centers for Disease Control and Prevention. Pink Book. https://www.cdc.gov/vaccines/pubs/pinkbook/vac-admin.html
- Centers for Disease Control and Prevention. Pandemic guidance. https://www.cdc.gov/vaccines/pandemic-guidance/index.html
References
- Centers for Disease Control and Prevention, Advisory Committee on Immunization Practices. ACIP vaccine recommendations and guidelines. Published July 16, 2013. https://www.cdc.gov/vaccines/hcp/acip-recs/index.html . Accessed March 1, 2021.
- Centers for Disease Control and Prevention. Birth-18 years immunization schedule. Published February 3, 2020. https://www.cdc.gov/vaccines/schedules/hcp/imz/child-adolescent.html . Accessed November 21, 2020.
- American Academy of Pediatrics. Immunization schedule. https://www.aap.org/en-us/advocacy-and-policy/aap-health-initiatives/immunizations/Pages/Immunization-Schedule.aspx . Accessed January 9, 2021.
- Centers for Disease Control and Prevention. ACIP general best practice guidelines for immunization. Published November 20, 2020. https://www.cdc.gov/vaccines/hcp/acip-recs/general-recs/index.html . Accessed January 9, 2021.
- US Department of Health and Human Services, Office of Disease Prevention and Health Promotion. Healthy People 2030 : Diabetes. https://health.gov/healthypeople/objectives-and-data/browse-objectives/diabetes . Accessed March 1, 2021.
- Santoli JM, Lindley MC, DeSilva MB, et al. Effects of the COVID-19 pandemic on routine pediatric vaccine ordering and administration – United States, 2020. MMWR Morb Mortal Wkly Rep 2020;69(19):591-3.
- Department of Health and Human Services. HHS expands access to childhood vaccines during COVID-19 pandemic. https://www.hhs.gov/about/news/2020/08/19/hhs-expands-access-childhood-vaccines-during-covid-19-pandemic.html . Accessed January 6, 2021.
- Ezeanolue E, Harriman K, Hunter P, Kroger A, Pellegrini C. General best practice guidelines for immunization. Best practices guidance of the Advisory Committee on Immunization Practices. www.cdc.gov/vaccines/hcp/acip-recs/generalrecs/downloads/general-recs.pdf . Accessed January 6, 2021.
- McKesson. Building a patient-centered pharmacy immunization service. www.mckesson.com/Blog/Building-a-Patient-Centered-Pharmacy-Immunization-Service/ . Accessed August 18, 2021.
- Centers for Disease Control and Prevention. VFC eligibility criteria. Published December 17, 2014. https://www.cdc.gov/vaccines/programs/vfc/providers/eligibility.html . Accessed January 9, 2021.
- Vogt TM, Zhang F, Banks M, et al. Provision of pediatric immunization services during the COVID-19 pandemic: an assessment of capacity among pediatric immunization providers participating in the Vaccines for Children Program – United States, May 2020. MMWR Morb Mortal Wkly Rep 2020;10;69(27):859-63. https://www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6927a2-H.pdf


Page 11.1: Soil Profiles and Processes
What is Soil?
The word “ soil ” has been defined differently by different scientific disciplines. In agriculture and horticulture, soil generally refers to the medium for plant growth, typically material within the upper meter or two (Figure \(\PageIndex{1}\)). We will use this definition in this chapter. Soil consists predominantly of mineral matter, but also contains organic matter ( humus ) and living organisms. The pore spaces between mineral grains are filled with varying proportions of water and air.
Figure \(\PageIndex{1}\). Soil Profile. Photograph shows a soil profile from South Dakota with A, E, and Bt horizons. The yellow arrows symbolize translocation of fine clays to the Bt horizon. The scale is in feet. Source: University of Idaho and modified by D. Grimley.In common usage, the term soil is sometimes restricted to only the dark topsoil in which we plant our seeds or vegetables. In a more broad definition, civil engineers use the term soil for any unconsolidated (soft when wet) material that is not considered bedrock. Under this definition, soil can be as much as several hundred feet thick! Ancient soils, sometimes buried and preserved in the subsurface, are referred to as paleosols (Figure \(\PageIndex{2}\)) and reflect past climatic and environmental conditions.
Importance of Soil
Soil is important to our society primarily because it provides the foundation of agriculture and forestry. Of course, soil is also a critical component for terrestrial ecosystems, and thus important to animals, plants, fungi, and microorganisms.
Figure \(\PageIndex{2}\). Modern versus Buried Soil Profiles. A buried soil profile, or paleosol (above geologist ‘s head), represents soil development during the last interglacial period. A modern soil profile (Alfisol) occurs near the land surface. Source: D. Grimley.Soil plays a role in nearly all biogeochemical cycles on the Earth’s surface. Global cycling of key elements such as carbon (C), nitrogen (N), sulfur (S), and phosphorous (P) all pass through soil. In the hydrologic cycle, soil helps to mediate the flow of precipitation from the surface into the groundwater. Microorganisms living in soil can also be important components of biogeochemical cycles through the action of decomposition and other processes such as nitrogen fixation.
Soil Forming Factors
The fundamental factors that affect soil genesis can be categorized into five elements: climate, organisms, relief, parent material, and time. One could say that the relief, climate, and organisms dictate the local soil environment and act together to cause weathering and mixing of the soil parent material over time. As soil is formed it often has distinct layers, which are formally described as “horizons.” Upper horizons (labeled as the A and O horizons) are richer in organic material and so are important in plant growth, while deeper layers (such as the B and C horizons) retain more of the original features of the bedrock below (Figure \(\PageIndex{3}\)).
Figure \(\PageIndex{3}\). This images shows the different horizons, or layers, in soil. This work by Wilsonbiggs is licensed under CC BY-SA 4.0Climate
The role of climate in soil development includes aspects of temperature and precipitation. Soils in very cold areas with permafrost conditions tend to be shallow and weakly developed due to the short growing season. Organic rich surface horizons are common in low-lying areas due to limited decomposition. In warm, tropical soils, soils tend to be thicker, with extensive leaching and mineral alteration. In such climates, organic matter decomposition and chemical weathering occur at an accelerated rate.
Organisms
Animals, plants, and microorganisms all have important roles in soil development processes, in providing a supply of organic matter, and/or in nutrient cycling. Worms, nematodes, termites, ants, gophers, moles, etc. all cause considerable mixing of soil and help to blend soil, aerate and lighten the soil by creating pores (which help store water and air).
Plant life provides organic matter to soil and helps to recycle nutrients with uptake by roots in the subsurface. The type of plant life that occurs in a given area, such as types of trees or grasses, depends on the climate, along with parent material and soil type. With the annual dropping of leaves and needles, trees tend to add organic matter to soil surfaces, helping to create a thin, organic-rich A or O horizon over time. Grasses, on the other hand, have a considerable root and surface masses that add to the soil each fall for annuals and short-lived perennials. For this reason, grassland soils have much thicker A horizons with higher organic matter contents, and are more agriculturally productive than forest soils.
Relief (Topography and Drainage)
The local landscape can have a surprisingly strong effect on the soils that form on site. The local topography ( relief ) can have important microclimatic effects as well as affecting rates of soil erosion. In comparison to flat regions, areas with steep slopes overall have more soil erosion, more runoff of rainwater, and less water infiltration, all of which lead to more limited soil development in very hilly or mountainous areas. In the northern hemisphere, south-facing slopes are exposed to more direct sunlight angles and are thus warmer and drier than north-facing slopes. The cooler, moister north-facing slopes have a more dynamic plant community due to less evapotranspiration and, consequently, experience less erosion because of plant rooting of soil and have thicker soil development.
Soil drainage affects organic matter accumulation and preservation, and local vegetation types. Well-drained soils, generally on hills or sideslopes, are more brownish or reddish due to conversion of ferrous iron (Fe 2+ ) to minerals with ferric (Fe 3+ ) iron. More poorly drained soils, in lowland, alluvial plains or upland depressions, tend more be more greyish, greenish-grey (gleyed), or dark colored, due to iron reduction (to Fe 2+ ) and accumulation and preservation of organic matter in areas tending towards anoxic. Areas with poor drainage also tend to be lowlands into which soil material may wash and accumulate from surrounding uplands, often resulting in overthickened A or O horizons. In contrast, steeply sloping areas in highlands may experience erosion and have thinner surface horizons.
Parent Material
The parent material of a soil is the material from which the soil has developed, whether it be river sands, shoreline deposits, glacial deposits, or various types of bedrock. In youthful soils, the parent material has a clear connection to the soil type and has significant influence. Over time, as weathering processes deepen, mix, and alter the soil, the parent material becomes less recognizable as chemical, physical, and biological processes take their effect. The type of parent material may also affect the rapidity of soil development. Parent materials that are highly weatherable (such as volcanic ash) will transform more quickly into highly developed soils, whereas parent materials that are quartz-rich, for example, will take longer to develop. Parent materials also provide nutrients to plants and can affect soil internal drainage (e.g. clay is more impermeable than sand and impedes drainage).
Time
In general, soil profiles tend to become thicker (deeper), more developed, and more altered over time. However, the rate of change is greater for soils in youthful stages of development. The degree of soil alteration and deepening slows with time and at some point, after tens or hundreds of thousands of years, may approach an equilibrium condition where erosion and deepening (removals and additions) become balanced. Young soils (< 10,000 years old) are strongly influenced by parent material and typically develop horizons and character rapidly. Moderate age soils (roughly 10,000 to 500,000 years old) are slowing in profile development and deepening, and may begin to approach equilibrium conditions. Old soils (>500,000 years old) have generally reached their limit as far as soil horizonation and physical structure, but may continue to alter chemically or mineralogically.
Soil development is not always continual. Geologic events can rapidly bury soils (landslides, glacier advance, lake transgression), can cause removal or truncation of soils (rivers, shorelines) or can cause soil renewal with additions of slowly deposited sediment that add to the soil (wind or floodplain deposits). Biological mixing can sometimes cause soil regression, a reversal or bump in the road for the normal path of increasing development over time.
Contributors and Attributions
- Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0. Modified from the original by Matthew R. Fisher.