(jel´; kldbomacmz´) a classification of immune mechanisms of tissue injury, comprising four types of hypersensitivity reactions : type I, immediate hypersensitivity reactions, mediated by interaction of immunoglobulin E (IgE) antibody and antigen and release of histamine and other mediators; type II, antibody-mediated hypersensitivity reactions, due to antibody-antigen interactions on cell surfaces; type III, immune complex–mediated hypersensitivity reactions, local or general inflammatory responses due to formation of circulating immune complexes and their deposition in tissues; and type IV cell-mediated hypersensitivity reactions, delayed hypersensitivity reactions initiated by sensitized T lymphocytes either by release of lymphokines or by T-cell–mediated cytotoxicity.