Allergy Tests Health Article

Definition

Allergy tests may be of two general types. In vivo tests that measure the immune response to an agent called an allergen that induces an allergic (atopic) reaction, and in vitro tests that measure the antibodies that mediate an allergic response. Such antibodies are those of the immunoglobulin E class (IgE) which have epsilon heavy chains which attach to mast cells.

Purpose

Allergy tests are performed to determine the cause of a person's allergic reaction. An allergic reaction is caused by the production of specific IgE antibodies against one or more antigens. Those antigens that elicit IgE production are termed allergens and are usually harmless substances. Some common allergens are described below.

• house dust mites and their waste
• hair, dander, and saliva of animals with fur or feathers
• cockroaches and their waste
• weed, grass, and tree pollens
• mold and mildew spores
• stinging insects such as bees, wasps, yellow jackets, hornets, and fire ants
• drugs such as penicillin, and sulfa drugs
• foods such as eggs, milk, nuts, and seafood
• ingredients found in dyes, cosmetics, and latex

Precautions

While allergy tests are safe for most people, there is always a possibility that the subject will experience a severe allergic reaction to an allergen used for in vivo testing. For this reason, the subject should be exposed to the lowest dose of allergen in the first phase of testing. In vitro blood tests are particularly appropriate for persons who have a history of severe allergy. In vitro testing may also be more appropriate for persons taking antihistamines, neuroleptics, or antidepressants, and for those with skin conditions all of which may interfere with skin test results.

Description

An allergic reaction is caused when a person's immune system produces IgE antibodies in response to a foreign antigen (allergen). IgE molecules are tightly bound to the surface of mast cells (and basophils in blood). These cells contain granules that have a high concentration of histamine and other substances that are responsible for the allergic reaction. Upon subsequent exposure to the same antigen, an immediate (type 1) hypersensitivity reaction called the atopic or allergic reaction ensues. The allergen binds to the IgE and the crosslinking of antigen and antibody molecules causes the mast cell to degranulate. The histamine and other allergic mediators are released and cause local swelling (edema) and redness (vasodilation). These reactions occur immediately and may be sufficient in intensity to cause constriction of the bronchi and shock. Such a systemic response to an allergen is called an anaphylactic reaction. Allergens most often responsible for anaphylactic reactions are insect bites and penicillin in persons who are allergic to these agents.

Allergies may be seasonal or chronic depending upon the exposure to the allergen. An allergy may be influenced by factors such as emotional stress, fatigue, infection, air pollution, and weather changes that can cause day to day variation in the severity of symptoms. These triggering factors add to what doctors call the "allergic load," the amount of allergens the body can tolerate at any given time without the occurrence of symptoms.

Allergies can manifest themselves in several ways. Conditions commonly caused by allergies include rhinitis, asthma, contact dermatitis, allergic gastritis, urticaria (hives), and conjunctivitis. Allergy tests are used to identify the allergen(s) responsible for the allergy. Once the allergen is known, avoidance and desensitization (subcutaneous injection of allergen extract) can be added to the treatment modality.