type IV hypersensitivity, a reaction initiated by antigen-specific T lymphocytes. Unlike hypersensitivity reactions mediated by antibodies, this type takes one or more days to develop, and the hypersensitivity can be transferred by lymphocytes but not by serum. The term is often equated with delayed hypersensitivity reactions that are cytokine-mediated (as contrasted with direct cytolysis). Also called cell-mediated hypersensitivity, T cell-mediated hypersensitivity reaction.
