Cell-mediated (Type) hypersensitivityIt is initiated by activated T-lymphocytes.It includes 1. Delayed-type hypersensitivity - mediated by CD-4 T cells. 2. T-cell mediated cytotoxicity - mediated by CDS- T cells. 1. Delayed-type hypersensitivityIn delayed hypersensitivity reactions, CD-4 T cells and macrophages are primarily involved. The term delayed is used to differentiate it from a secondary' cellular response, which appears at 48-72 hours after antigen exposure and from an immediate hypersensitivity response, which generally appears within minutes, Delayed hypersensitivity is a major mechanism of defence against various intracellular pathogens, including mycobacteria, fungi and certain parasites.PathophysiologyDelayed hypersensitivity is characterized by the formation of granuloma - a localized mass that contains activated macrophages (epithelioid) cells surrounded by colour of lymphocytes and giant cells. It is also called granulomatous Inflammation. The cellular events start when an antigen (eg tuberculin antigen) on antigen presenting cell comes in contact with naive CD-4 T cells → naive CD-4 T cells recognize this peptide antigen in association with MHC – II and differentiate into T4-l cells. Induction of T4-l cells is of central importance because the expression of delayed hypersensitivity largely depends on the cytokines secreted by T4-l cells.2. T-cell mediated cytotoxicityIn this type of hypersensitivity, cytotoxic T cells kill virus-infected or tumor cells.