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Hundreds or even thousands of ordinary substances can trigger allergic reactions. These are called "allergens." Among the most common are plant pollens, molds, household dust (dust mites), animal dander, industrial chemicals, foods, medicines and insect stings.

An allergic reaction may occur anywhere in the body, but usually appears in the skin, eyes, lining of the stomach, nose, sinuses, throat and lungs -- places where special immune system cells are stationed to fight off invaders that are inhaled, swallowed or come in contact with the skin.

Allergies can affect anyone, regardless of age, gender, race or socioeconomic status. While it's true that allergies are more common in children, they can occur for the first time at any age or, in some cases, recur after many years of remission.

Although the exact genetic factors are not yet understood, the tendency to allergies, as well as to allergic disease, is linked to heredity.

The best first step in the diagnosis of allergies is a thorough health history and physical examination. If you have allergy symptoms that occur in association with exposure to certain things, that is highly significant. Allergy diagnostic tests, such as skin tests or blood tests, provide similar information and merely confirm what your health history tells the doctor. If your doctor were to rely exclusively on the results of skin or blood tests (without history and physical examination), you could be diagnosed as having an allergic problem that you don't necessarily have.

Skin tests, in most situations, are preferable because (1) the results are available immediately, (2) they are less expensive and (3) they are more sensitive to subtle allergies.

A blood test is appropriate in certain situations, particularly when you (1) cannot suspend antihistamine therapy which can inhibit skin tests, (2) have widespread skin disease making skin testing difficult, (3) are so sensitive to the allergen that the test might be risky or (4) cannot be skin tested for some other reason.

Like all medical treatments, allergy shots (immunotherapy) can have side effects. Your doctor will discuss this with you in detail. Under no circumstances should you consider allergy shots without at least an attempt at avoidance of the troublesome allergen. For instance, cat allergy shots are no substitute for cat avoidance. Some allergens, though, such as grass pollen, are almost unavoidable.

Why is it that frequent exposure to an allergen can increase sensitivity and cause allergic reaction, yet repeated exposure to an allergen in allergy shots helps build ups immunity?

Regularly scheduled, repeated exposure to small amounts of an allergen can lead to immunity, whereas infrequent and erratic exposure does not confer immunity but increases the likelihood of producing allergen sensitization.

Irregular exposure to allergens can lead to the production of antibodies (called IgE-mediated antibodies). The presence of these antibodies, when exposed to an allergen can lead to an allergic reaction.

In allergy shots or immunotherapy, the allergen exposure is closely regulated and given on a scheduled basis. Small amounts of allergens are given over a period of time to build up to maintenance doses. This leads to the production of blocking antibodies (called IgG antibodies) and a decrease in the level of allergic or IgE-mediated antibodies.

Q. How can asthma be controlled when one is also suffering from GERD?

A. GERD, or Gastroesophageal reflux disease, triggers or worsens respiratory symptoms in a number of people with asthma. Once the GERD is controlled, asthma often improves – leading to a reduction in medication required and allowing easier breathing.

GERD is a common condition in which the acid from the stomach flows up the esophagus causing irritation and inflammation. This is often felt as a burning sensation in the upper abdomen and can lead to a bad taste in the mouth and a sense of burning in the chest if the fluid rises high enough. Since the nerves in the lower esophagus are connected to the nerves in the lungs, it is common for acid reflux to trigger asthma symptoms so it is important to control the reflux as much as possible.

The most important way to control reflux is to first see a doctor to make sure that reflux is the correct diagnosis, as other conditions can mimic it. The treatment for GERD itself is the same as if one did not have asthma. In many cases a simple trial of an acid reducer such as ranitidine, which is available over the counter, might control the symptoms. Stronger medications called proton pump inhibitors also can reduce acid secretion. From a dietary standpoint, it helps to avoid eating meals for several hours before bedtime (avoiding large evening meals) and to reduce fat intake. Alcohol consumption in excess and caffeine can also predispose to acid reflux. Sleeping on several pillows with the upper body propped up to let gravity keep the fluid down may help. Weight loss for those who are overweight is recommended, as weight gain can worsen reflux.

It is also important to assess whether certain asthma medications may be worsening GERD. Asthma medications that could increase reflux include theophyllines and oral steroids, and should be used only if there are no other alternatives. For most who suffer from both conditions, asthma can be controlled with the usual asthma therapies along with treatment of GERD.

If these simple measures don't work, then more extensive evaluation and treatment may be needed. Sometimes, medical therapy is not adequate and GERD will require a surgical repair. Further evaluation should be done under the supervision of a qualified physician.

Asthma is a chronic, inflammatory lung disease characterized by recurrent breathing problems. People with asthma have acute episodes when the air passages in their lungs get narrower, and breathing becomes more difficult.

The problem is an oversensitivity of the lungs and airways, which overreact to certain "triggers" and become inflamed and clogged.

The cause of the lung abnormality that is asthma is not yet known. Through research, scientists have established that the disease is a special type of inflammation of the airway that leads to contraction of airway muscle, mucus production and swelling in the airways. The airways become overly responsive to environmental changes. The result is wheezing and coughing.

As yet there is no cure for asthma, but asthma can be controlled with proper treatment. People with asthma can use medicine prescribed by their physician to prevent or relieve their symptoms, and they can learn ways to manage episodes. They also can learn to identify and avoid the things that trigger an episode. By educating themselves about medications and other asthma management strategies, most people with asthma can gain control of the disease and live an active life.

No. Although episodes of asthma can sometimes be brought on by strong emotions, it is important to know that asthma is not the result of emotional factors such as a troubled parent-child relationship. Years ago, people more commonly believed that asthma was "all in one's head" and therefore not a real illness. Physicians and other medical scientists today know that this is wrong.

Asthma is sometimes hard to diagnose because it can resemble other respiratory problems such as emphysema, bronchitis and lower respiratory infections. For that reason, asthma is underdiagnosed – that is, many people with the disease do not know they have it and therefore are never treated. Sometimes the only symptom is a chronic cough, especially at night. Or, coughing or wheezing may occur only with exercise. Some people mistakenly think they are having recurrent bronchitis, since respiratory infections usually settle in the chest in a person predisposed to asthma.

To diagnose asthma and distinguish it from other lung disorders, physicians rely on a combination of medical history, a thorough physical examination, and certain laboratory tests. These tests include spirometry (using an instrument that measure the air taken into and out of the lungs), peak flow monitoring (another measure of lung function), chest X-rays and sometimes blood and allergy tests.

An asthma episode feels somewhat like taking deep breaths of very cold air on a winter day. Breathing becomes harder and may hurt, and there may be coughing. Breathing may make a wheezing or whistling sound.

These problems occur because the airways of the lungs are getting narrower. The muscles that surround the airways tighten, the inner lining of the airways swells and pushes inward, and the membranes that line the airways secrete extra mucus, which can form plugs that further block the air passages. The rush of air through the narrowed airways produces the wheezing sounds that are typical of asthma.

There are several asthma support groups. One national organization is the Allergy and Asthma Network Mothers of Asthmatics. Another is the Asthma and Allergy Foundation of America. There also is a Food Allergy and Anaphylaxis Network.