type I hypersensitivity, hypersensitivity that occurs rapidly, within several minutes, on reexposure to an antigen. Reexposure cross-links the immunoglobulin E present on basophils and mast cells, resulting in degranulation and the secretion of products such as histamine and leukotrienes. Anaphylaxis is a particularly severe type I hypersensitivity reaction. Also called immediate hypersensitivity reaction. See anaphylactic hypersensitivity.
