Allergic diseases result from IgE-mediated immune responses to foreign protein (allergens). The majority of such reactions are IgE-mediated (type I) reactions. Individuals who develop such reactions are allergic. Those predisposed on a genetic basis to synthesize IgE to environmental allergens are atopic. Most allergic reactions are precipitated when a specific allergen aggregates several IgE molecules attached to IgE receptors on the surfaces of mast cells and basophils. Chemical mediators are released which lead to the immediate signs and symptoms associated with allergic diseases including hives, asthma, and anaphylaxis. More prolonged reactions follow if significant numbers of other cells including eosinophils, macrophages, and lymphocytes are drawn into sites of mast cell activation. Therefore, allergic diseases result from a complex interplay of immune cells, foreign proteins, and tissue inflammation.