a local or general reaction of an organism following contact with a specific allergen to which it has been previously exposed and sensitized; immunologic interaction of endogenous or exogenous antigen with antibody or sensitized lymphocytes gives rise to inflammation or tissue damage. Allergic reaction’s are classified into four major types: type I, anaphylactic and IgE dependent; type II, cytotoxic; type III, immune-complex mediated; type IV, cell-mediated (delayed). Syn: hypersensitivity reaction.