What Is an Allergy?

An allergy is a physiological reaction caused when the immune system mistakenly identifies a normally harmless substance as damaging to the body.

Normally, the human body defends itself against harmful substances such as viruses or bacteria, but sometimes the defenses aggressively attack usually innocuous substances such as dust, mold, or pollen.

The immune system generates large amounts of the antibodies called immunoglobin E (IgE), a complex chemical weapon, to attack and destroy the supposed enemy. Each IgE antibody specifically targets a particular allergen—the substance that causes the allergy. In this disease-fighting process, inflammatory chemicals such as histamines, cytokines, and leukotrienes are released or produced, and some unpleasant (and, in extreme cases, life-threatening) symptoms may be experienced by an allergy-prone person.