allergic disorder in which large, localized, painless swellings similar to hives appear under the skin. The swelling is caused by massive accumulation of fluid (edema) following exposure to an allergen (a substance to which the person has been sensitized) or, in cases with a hereditary disposition, after infection or injury. The reaction appears suddenly and persists for a few hours or days, occurring most often on the face, hands, feet, genitals, and mucous membranes.

A number of foods and drugs can precipitate allergic angioedema. The condition can usually be controlled with antihistamines (antihistamine) or epinephrine (epinephrine and norepinephrine) and seldom poses serious danger to the affected person. In hereditary angioedema, caused by a defect in the immune system, swellings in the intestinal tract may produce pain, vomiting, or diarrhea, and edema of the larynx may cause death by asphyxiation. Hereditary angioedema usually first appears in late adolescence or early adulthood. It cannot be controlled by the same methods as allergic angioedema; however, drugs to treat this form have also been developed.