Updated June 10, 2015.

Definition: A foreign substance which causes the immune system to produce immunoglobulin E antibody (IgE) against the substance. The IgE binds to cells such as mast cells and basophils. Upon re-exposure, the same foreign substance binds to IgE, causing chemical mediators to be released from the mast cells and basophils, resulting in vasodilation, itching, sneezing, wheezing and other allergic symptoms.