Allergy testing: body’s timing reveals clues
Dear Dr. K: Until my husband recently came to see you I didn’t realize there was any other type of allergy tests than the prick tests I received. Can you explain?
The skin test your husband received was patch-testing to try to ferret the cause of his skin rash. The test is designed to determine if he is sensitive to any contact allergens. The patches are applied and left in place for several days before being read. The test investigates a type of allergy called delayed hypersensitivity (DH).
The tests you received were scratch tests for food and airborne allergies. These assess for a type of allergy called immediate hypersensitivity (IH), which is why the tests are read within minutes of being applied.
These two distinct types of allergies have different physiologic causes. IH is mediated* by an allergic protein called IgE, and accounts for most common allergies such as hay fever, asthma, hives and food allergy. The reactions (and also the tests) are “immediate” because the IgE is a tiny protein distributed throughout the body and bloodstream and can react right away. DH, on the other hand, is mediated* by white blood
cells called lymphocytes that must migrate from the blood stream into the tissue (such as the skin) that is experiencing the allergy. This migration is a bit ponderous, hence, the delayed nature of the allergy (and true results of the test). The tests are read twice: upon removal of the patches and then, 48 hours later.
Familiar examples of this type are rashes from poison ivy or from mango.
Allergy Primer
*From the medical dictionary: Mediate: to serve as an intermediary substance.
When you are tested for either type of allergy, what is revealed is that your body has generated these intermediary defenses which help us discern what made you react.