Immunoglobulin E (IgE) is the antibody class central to type I hypersensitivity (allergic) reactions. When an allergen is first encountered, the immune system produces allergen-specific IgE antibodies that bind to mast cells and basophils. On re-exposure, the allergen cross-links these IgE molecules, triggering the release of histamine and other inflammatory mediators — producing the symptoms of allergy.

Total serum IgE is elevated in allergic diseases including hay fever, asthma, eczema, food allergy, and allergic rhinitis. Very high total IgE can also indicate parasitic infection, certain immunodeficiency conditions (such as hyper-IgE syndrome), or IgE myeloma. Specific IgE testing against individual allergens (e.g. grass pollen, peanut) provides more clinically actionable allergy information.