What Is Allergy?

An allergy is a peculiar reaction to a substance than is not harmful or irritating to normal, non-allergic people. "Allergy," however, is an often misunderstood term. Most people believe that an allergy refers to any uncomfortable reaction to a substance that is inhaled, ingested, or applied to the skin. Symptoms that are attributed to allergy include sneezing, wheezing, nasal irritation, cough, runny and/or stuffy nose, heartburn, bloating, diarrhea, and a variety of skin rashes.

The main confusion is that some substances can cause these symptoms in a non-allergic way. Examples of non-allergic reactions would be inhaling cigarette smoke (causing sneezing because of simple irritation) and ingesting milk (causing bloating because of a deficiency of an enzyme that helps digest sugars in the milk). Further complicating the issue is that a substance like milk can also cause true allergic reactions including hives and wheezing.

In fact, "allergy" implies that the body's immune system is responding to a substance, or allergen, in such a way that it leads to some of the symptoms mentioned above. This occurs when the immune system sends white blood cells (as well as other cells and chemical mediators) to the site of the body where it encounters an allergen. These cells and chemicals cause the changes in the tissues that lead to allergic-type symptoms. Examples of allergens include tree pollen, cat dander, dust mites, and several foods.

A requirement of such an allergic immune response is that the immune system be able to recognize a substance as being foreign, not normally present in the body. Put simply, the substance must contain molecules with certain characteristics (having a protein or large carbohydrate structure, for example) to enable it to be recognized by the immune system and give rise to an allergic response. This explains why the runny nose caused by cold air in some is not a true allergic phenomenon. People with allergic inflammation in their noses, however, are often more troubled by irritants such as smoke. A useful analogy is the following: Salt poured on intact, healthy skin causes no discomfort, but if you pour salt on an open wound it is quite painful.

Why allergy occurs in the first place is still a mystery. Some believe that allergy is simply a mistake of the immune system. That is, the immune system may "believe" that an allergen is an infectious organism such as a bacteria or virus. Consequently, the immune systems sends those white blood cells (such as lymphocytes) and chemicals to the tissues to ward off the infection. Nasal congestion, for instance, might represent the immune system's attempt to restrict the allergen (thinking it's a virus or bacteria) from gaining deeper entry into the body.

The immune system is also charged with defending the body from cancer, and an over enthusiastic one may also lead to allergy. Alternatively, the body's immune system might be missing certain immune suppressor functions. It should be mentioned that the overwhelming majority of people with allergies have no defect in the ability to fight off infections or defend against cancer.

Researchers have noted that there is an increase in the proportion of people suffering from allergies. Some believe that air pollution and heavier exposure to indoor allergens (spending more time inside tightly insulated homes) is to blame. Another interesting theory is that allergies are our societies trade off for being so efficiently sanitary. That is, our immune systems are not as busy fighting off genuine infections as much as before, with the consequence being more frequent mistaken allergic immune responses. This theory is interesting in light of the lower incidence of allergy seen in parts of the world with more exposure to parasitic infections (the Nile region, e.g.).

Finally, the question is what to do about treating allergic symptoms? First, it is important to determine what a person may be allergic to. The diagnosis can sometimes be made by the patient's history alone, and but specific tests (skin-prick or RAST blood tests) are occasionally needed to confirm the diagnosis. The next step is to avoid exposure to the allergens. For food allergies this is the only scientifically proven treatment. But for airborne allergens, medications can be very helpful since it may be impossible to totally avoid exposure. In addition to medications, immunotherapy (allergen injections) can be an effective remedy (see our list of Frequently Asked Questions on Immunotherapy).

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