What Exactly is an Allergy?

An allergy is a reaction of the immune system to normally harmless substances that don’t cause problems for most people. These substances – which include pet dander, certain foods, molds and pollens – cause a false-alarm reaction because the immune system treats it as an invader, generating large amounts of the disease-fighting protein immunoglobulin E (IgE), a type of antibody.

Normally present at very low levels in the body, IgE is found in larger quantities in children and adults with allergies. If you’re allergy-prone, the first time you’re exposed to an allergen, you begin to make large amounts of the corresponding IgE antibody. These IgE molecules attach to the surfaces of mast cells (in tissue) or basophils (in blood). Mast cells are particularly plentiful in the lungs, skin, tongue and lining of the nose and intestinal tract, places where the outside of your body meets the inside. When the IgE molecules latch on, they activate the mast cells, which begin spitting out the chemicals (histamine, cytokines and leukotrienes) that cause the allergic response, and the wheezing, sneezing, runny eyes and itching.

Return to our Special Report on Allergies