https://doi.org/10.1351/goldbook.13255
Reaction provoked by re-exposure to an antigen (allergen) that causes plasma cells to secrete immunoglobulin E (IgE) which binds to Fc receptors on the surface of tissue mast cells and blood basophils, sensitizing them, so that later exposure to the same allergen cross-links the bound IgE, leading to degranulation and the secretion of pharmacologically active mediators such as histamine, leukotriene, and prostaglandin that act immediately on the surrounding tissues.
Note: The principal effects of these products are vasodilation and smooth-muscle contraction. The reaction may be either local or systemic. Symptoms vary from mild irritation to sudden death from anaphylactic shock.