IgE

IgE, or immunoglobulin E, is one of the five major antibody isotypes produced by plasma cells. It circulates at very low concentrations in the serum and has a specialized role in defense against helminth parasites as well as in allergic reactions. A single IgE molecule is a monomer composed of two heavy epsilon chains and two light chains, with the Fc portion adapted to engage specific receptors and the Fab regions capable of recognizing antigens.

IgE production is driven by Th2 responses. Upon exposure to certain antigens, interleukin-4 and interleukin-13 promote class-switch recombination in B cells, generating IgE rather than other isotypes. Secreted IgE binds with high affinity to FcεRI, the high-affinity receptor on mast cells and basophils, effectively arming these cells for subsequent encounters.

On re-exposure to the allergen, cross-linking of IgE bound to FcεRI triggers rapid degranulation of mast cells and basophils, releasing histamine, leukotrienes, prostaglandins, and other mediators. This immediate hypersensitivity reaction causes symptoms such as itching, swelling, rhinorrhea, bronchoconstriction, and, in severe cases, anaphylaxis. A later inflammatory phase can involve eosinophils and other cells.

FcεRI is the primary high-affinity IgE receptor and is expressed on mast cells and basophils; FcεRII (CD23) is a lower-affinity receptor found on B cells and some other immune cells and can modulate IgE synthesis. Serum IgE levels are normally low but may be elevated in allergic diseases or certain parasitic infections.

Clinically, IgE measurements assist in allergy evaluation, and therapies targeting IgE, such as omalizumab, reduce free IgE and down-regulate FcεRI expression on effector cells. IgE also contributes to protective immunity against certain parasites, illustrating its dual role in defense and disease. IgE was identified in 1966 by Ishizaka and Ishizaka.