American Medical Association Family Medical Guide - American Medical Association [624]
An allergic reaction
During an allergic reaction, the immune system responds to a normally harmless substance as if it were a harmful invader. During the first exposure to the substance (referred to as an allergen), the immune system produces antibodies that attach themselves to cells called mast cells. The next time the allergen enters the body, it bonds with the antibodies. This bond stimulates the mast cells to release many powerful chemicals, including histamine. The release of these chemicals from the mast cells causes the characteristic symptoms of an allergic reaction.
Diagnosing Allergies
Doctors and allergists use the following tests to help diagnose most types of allergies.
Skin Tests
Skin tests (also called patch tests or scratch tests) are most often used to test for allergies to airborne allergens or particular foods. In a scratch test, a small amount of the suspected allergen is pricked or scratched into the skin. A safer alternative (which reduces the risk of the allergen getting into the bloodstream) is a patch test, in which the suspected allergen is applied to a patch taped to the skin. Skin testing, especially scratch testing, is not usually performed on anyone who has had a life-threatening reaction to an allergen or who has a skin disorder (such as dermatitis) that could affect the results of the test. Any reaction (such as redness, swelling, or itching) usually indicates an allergic reaction to the substance being tested.
RAST Blood Test
The radioallergosorbent test (RAST) is the blood test most frequently used to detect all types of allergens. It can help identify the substance that is causing your allergic reaction by measuring the level of IgE antibodies in your blood after exposure to the substance. (IgE antibodies are proteins the body produces in most allergic reactions.) RAST can be done even if you are taking medications for the allergy or if you are having symptoms (such as dermatitis) that could alter the results of skin tests. A positive test result usually means that you are allergic to the substance being tested.
Treating Allergies
The best treatment for any allergy is complete avoidance of the allergen. Sometimes it is possible to make an affected person less sensitive to an allergen in a process called desensitization. Desensitization is sometimes used to treat the symptoms of airborne allergies such as pollen or dust mites, and severe reactions to wasp or bee stings. Doctors usually recommend desensitization only if a person has mild to moderate symptoms more than once a year, and if the symptoms cannot be controlled with medication or by avoiding the allergen.
For desensitization, which is done over a 3- to 5-year period, the doctor administers a series of injections containing a small, carefully measured amount of the allergen to stimulate the immune system to produce protective antibodies. The allergen is given in increasing doses over the 3- to 5-year period until a sufficient number of antibodies has been produced to prevent the body from reacting to the allergen or to make it react less strongly, causing fewer symptoms. Desensitization is not used in all cases because it is not effective for everyone and because it can cause a life-threatening allergic reaction (anaphylactic shock; see page 916) in people who have a severe allergy.
Allergies to Airborne Substances
Allergies to airborne substances are caused by contact with microscopic particles from pollen, mold, dust mites, smoke, or animal dander (tiny skin flakes) that are inhaled from the air. Airborne allergies include seasonal allergies such as hay fever (see next page). Some synthetic materials found in the home or workplace—such as paint, polyurethane, and artificial fibers—can