Gell and Coombs classification /jel; kōōmz/ , a classification of immune mechanisms of tissue injury, comprising four types of hypersensitivity reactions: type I (anaphylactic reactions), immediate hypersensitivity reactions mediated by interaction of immunoglobulin E antibody and antigen and release of histamine and other mediators; type II (cytotoxic reactions), antibody-mediated hypersensitivity reactions caused by antibody-antigen interactions on cell surfaces; type III (immune complex reactions), mediated hypersensitivity reactions, local or general inflammatory responses caused by 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.
