Classification of immune mechanisms of tissue injury into four types: type I, immediate-type hypersensitivity reactions, mediated by interaction of immunoglobulin E (IgE) antibody and antigen causing the release of histamine and other mediators; type II, antibody-mediated hypersensitivity reactions, due to antibody–antigen interactions on cell surfaces; type III, immune complex, local or general inflammatory responses due to formation of circulating immune complexes and their deposition in tissues; and type IV cell-mediated hypersensitivity reactions, initiated by sensitized T lymphocytes either by release of lymphokines or by T-cell-mediated cytotoxicity [see cytotoxic T lymphocyte (CTL)].