as intracellular bacteria) that can survive and replicate inside host cells, especially those pathogens that replicate even after being phagocytosed by macrophages. after the tcr of th1 cells recognize specific antigen peptide-bound mhc class ii molecules on macrophages, th1 cells 1) secrete ifn-γ and 2) upregulate the expression of cd40 ligand (cd40l), which binds to cd40 on macrophages. these 2 signals activate the macrophages and enhance their ability to kill intracellular pathogens through increased production of antimicrobial molecules such as nitric oxide (no) and superoxide (o2-). this enhancement of macrophages' antimicrobial ability by th1 cells is known as classical macrophage activation, and the activated macrophages are known as classically activated macrophages, or m1 macrophages. the m1 macrophages in turn upregulates b7 molecules and antigen presentation through mhc class ii molecules to provide signals that sustain t cell help. the activation of th1 and m1 macrophage is a positive feedback loop, with ifn-γ from th1 cells upregulating cd40 expression on macrophages; the interaction between cd40 on the macrophages and cd40l on t cells activate macrophages to secrete il-12; and il-12 promotes more ifn-γ secretion from th1 cells. the initial contact between macrophage antigen-bound mhc ii and tcr serves as the contact point between the two cells where most of the ifn-γ secretion and cd-40l on t cells concentrate to, so only macrophages directly interacting with th1 cells are likely to be activated. in addition to activating m1 macrophages, th1 cells express fas ligand (fasl) and lymphotoxin beta (lt-β) to help kill chronically infected macrophages that can no longer kill pathogens. the killing of chronically infected macrophages release pathogens to the extracellular space that can then be killed by other activated macrophages. th1 cells also help recruit more monocytes, the precursor to macrophages, to the infection site. th1 secretion tnf-α and lt-α to make blood vessels easier for monocytes to bind to and exit. th1 secretion of ccl2 as a chemoattractant for monocytes. il-3 and gm-csf released by th1 cells stimulate more monocyte production in the bone marrow. when intracellular pathogens cannot be eliminated, such as in the case of mycobacterium tuberculosis, the pathogen is contained through the formation of granuloma, an aggregation of infected macrophages surrounded by activated t cells. the macrophages bordering the activated lymphocytes often fuse to form multinucleated giant cells that appear to have increased antimicrobial ability due to their proximity to th1 cells, but over time, the cells in the center start to die and form necrotic tissue. alternative th2 cells play an important role in alternative macrophage activation as part of type 2 immune response against large extracellular pathogens like helminths. th2 cells secrete il-4 and il-13, which activate macrophages to become m2 macrophages, also known as alternatively activated macrophages. m2 macrophages express arginase-1, an enzyme that converts arginine to ornithine and urea. ornithine help increase smooth muscle contraction to expel the worm and also participates in tissue and wound repair. ornithine can be further metabolized to proline, which is essential for synthesizing collagen. m2 macrophages can also decrease inflammation by producing il-1 receptor antagonist (il-1ra) and il-1 receptors that do not lead to downstream inflammatory signaling (il-1rii). interactions with cd8+ cytotoxic t cells another part of the adaptive immunity activation involves stimulating cd8+ via cross presentation of antigens peptides on mhc class i molecules. studies have shown that proinflammatory macrophages are capable of cross presentation of antigens on mhc class i molecules, but whether macrophage cross-presentation plays a role in naïve or memory cd8+ t cell activation is still unclear. interactions with b cells macrophages have been shown to secrete cytokines baff and april, which are important for plasma cell isotype switching. april and il-6 secreted by macrophage precursors in the bone marrow help maintain survival of plasma cells homed to the bone marrow. subtypes siderophageanthracotic macrophagepigmented macrophages can be classified by the pigment type, such as for alveolar macrophages shown above (white arrows). a "siderophage" contains hemosiderin (also shown by black arrow in left image), while anthracotic macrophages result from coal dust inhalation (and also long-term air pollution). h&e stain. there are several activated forms of macrophages. in spite of a spectrum of ways to activate macrophages, there are two main groups designated m1 and m2. m1 macrophages: as mentioned earlier (previously referred to as classically activated macrophages), m1 "killer" macrophages are activated by lps and ifn-gamma, and secrete high levels of il-12 and low levels of il-10. m1 macrophages have pro-inflammatory, bactericidal, and phagocytic functions. in contrast, the m2 "repair" designation (also referred to as alternatively activated macrophages) broadly refers to macrophages that function in constructive processes like wound healing and tissue repair, and those that turn off damaging immune system activation by producing anti-inflammatory cytokines like il-10. m2 is the phenotype of resident tissue macrophages, and can be further elevated by il-4. m2 macrophages produce high levels of il-10, tgf-beta and low levels of il-12. tumor-associated macrophages are mainly of the m2 phenotype, and seem to actively promote tumor growth. macrophages exist in a variety of phenotypes which are determined by the role they play in wound maturation. phenotypes can be predominantly separated into two major categories; m1 and m2. m1 macrophages are the dominating phenotype observed in the early stages of inflammation and are activated by four key mediators: interferon-γ (ifn-γ), tumor necrosis factor (tnf), and damage associated molecular patterns (damps). these mediator molecules create a pro-inflammatory response that in return produce pro-inflammatory cytokines like interleukin-6 and tnf. unlike m1 macrophages, m2 macrophages secrete an anti-inflammatory response via the addition of interleukin-4 or interleukin-13. they also play a role in wound healing and are needed for revascularization and reepithelialization. m2 macrophages are divided into four major types based on their roles: m2a, m2b, m2c, and m2d. how m2 phenotypes are determined is still up for discussion but studies have shown that their environment allows them to adjust to whichever phenotype is most appropriate to efficiently heal the wound. m2 macrophages are needed for vascular stability. they produce vascular endothelial growth factor-a and tgf-β1. there is a phenotype shift from m1 to m2 macrophages in acute wounds, however this shift is impaired for chronic wounds. this dysregulation results in insufficient m2 macrophages and its corresponding growth factors that aid in wound repair. with a lack of these growth factors/anti-inflammatory cytokines and an overabundance of pro-inflammatory cytokines from m1 macrophages chronic wounds are unable to heal in a timely manner. normally, after neutrophils eat debris/pathogens they perform apoptosis and are removed. at this point, inflammation is not needed and m1 undergoes a switch to m2 (anti-inflammatory). however, dysregulation occurs as the m1 macrophages are unable/do not phagocytose neutrophils that have undergone apoptosis leading to increased macrophage migration and inflammation. both m1 and m2 macrophages play a role in promotion of atherosclerosis. m1 macrophages promote atherosclerosis by inflammation. m2 macrophages can remove cholesterol from blood vessels, but when the cholesterol is oxidized, the m2 macrophages become apoptotic foam cells contributing to the atheromatous plaque of atherosclerosis. role in muscle regeneration the first step to understanding the importance of macrophages in muscle repair, growth, and regeneration is that there are two "waves" of macrophages with the onset of damageable muscle use – subpopulations that do and do not directly have an influence on repairing muscle. the initial wave is a phagocytic population that comes along during periods of increased muscle use that are sufficient to cause muscle membrane lysis and membrane inflammation, which can enter and degrade the contents of injured muscle fibers. these early-invading, phagocytic macrophages reach their highest concentration about 24 hours following the onset of some form of muscle cell injury or reloading. their concentration rapidly declines after 48 hours. the second group is the non-phagocytic types that are distributed near regenerative fibers. these peak between two and four days and remain elevated for several days during while muscle tissue is rebuilding. the first subpopulation has no direct benefit to repairing muscle, while the second non-phagocytic group does. it is thought that macrophages release soluble substances that influence the proliferation, differentiation, growth, repair, and regeneration of muscle, but at this time the factor that is produced to mediate these effects is unknown. it is known that macrophages' involvement in promoting tissue repair is not muscle specific; they accumulate in numerous tissues during the healing process phase following injury. role in wound healing macrophages are essential for wound healing. they replace polymorphonuclear neutrophils as the predominant cells in the wound by day two after injury. attracted to the wound site by growth factors released by platelets and other cells, monocytes from the bloodstream enter the area through blood vessel walls. numbers of monocytes in the wound peak one to one and a half days after the injury occurs. once they are in the wound site, monocytes mature into macrophages. the spleen contains half the body's monocytes in reserve ready to be deployed to injured tissue. the macrophage's main role is to phagocytize bacteria and damaged tissue, and they also debride damaged tissue by releasing proteases. macrophages also secrete a number of factors such as growth factors and other cytokines, especially during the third and fourth post-wound days. these factors attract cells involved in the proliferation stage of healing to the area. macrophages may also restrain the contraction phase. macrophages are stimulated by the low oxygen content of their surroundings to produce factors that induce and speed angiogenesis and they also stimulate cells that re-epithelialize the wound, create granulation tissue, and lay down a new extracellular matrix. by secreting these factors, macrophages contribute to pushing the wound healing process into the next phase. role in limb regeneration scientists have elucidated that as well as eating up material debris, macrophages are involved in the typical limb regeneration in the salamander. they found that removing the macrophages from a salamander resulted in failure of limb regeneration and a scarring response. role in iron homeostasis main article: human iron metabolism as described above, macrophages play a key role in removing dying or dead cells and cellular debris. erythrocytes have a lifespan on average of 120 days and so are constantly being destroyed by macrophages in the spleen and liver. macrophages will also engulf macromolecules, and so play a key role in the pharmacokinetics of parenteral irons. the iron that is released from the haemoglobin is either stored internally in ferritin or is released into the circulation via ferroportin. in cases where systemic iron levels are raised, or where inflammation is present, raised levels of hepcidin act on macrophage ferroportin channels, leading to iron remaining within the macrophages. role in pigment retainment melanophage. h&e stain. melanophages are a subset of tissue-resident macrophages able to absorb pigment, either native to the organism or exogenous (such as tattoos), from extracellular space. in contrast to dendritic juncional melanocytes, which synthesize melanosomes and contain various stages of their development, the melanophages only accumulate phagocytosed melanin in lysosome-like phagosomes. this occurs repeatedly as the pigment from dead dermal macrophages is phagocytosed by their successors, preserving the tattoo in the same place. role in tissue homeostasis every tissue harbors its own specialized population of resident macrophages, which entertain reciprocal interconnections with the stroma and functional tissue. these resident macrophages are sessile (non-migratory), provide essential growth factors to support the physiological function of the tissue (e.g. macrophage-neuronal crosstalk in the guts), and can actively protect the tissue from inflammatory damage. nerve-associated macrophages nerve-associated macrophages or nams are those tissue-resident macrophages that are associated with nerves. some of them are known to have an elongated morphology of up to 200μm clinical significance due to their role in phagocytosis, macrophages are involved in many diseases of the immune system. for example, they participate in the formation of granulomas, inflammatory lesions that may be caused by a large number of diseases. some disorders, mostly rare, of ineffective phagocytosis and macrophage function have been described, for example. as a host for intracellular pathogens in their role as a phagocytic immune cell macrophages are responsible for engulfing pathogens to destroy them. some pathogens subvert this process and instead live inside the macrophage. this provides an environment in which the pathogen is hidden from the immune system and allows it to replicate. diseases with this type of behaviour include tuberculosis (caused by mycobacterium tuberculosis) and leishmaniasis (caused by leishmania