Type IV hypersensitivity is -

Ans. is 'c' i.e., Cell mediated Cell mediated (Type IV) hypersensitivity o It is initiated by activated T-lymphocytes. o It includes Delayed type hypersensitivity - mediated by CD-4 T cells. T-cell mediated cytotoxicity - mediated by CD8- T cells. 1. Delayed type hypersensitivity In delayed hypersensitivity reactions, CD-4 T cells and macrophages are primarily involved. o 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 hypersinsitivity is a major mechanism of defence against various intracellular pathogens, including mycobacteria, fungi and ceain parasites. Pathophysiology Delayed hypersensitivity is characterized by formation of granuloma - a localized mass that contains activated macrophages (epitheloid) cells surrounded by collor of lymphocytes and giant cells. o It is also called granulomatous inflammation. o The cellular events stas when antigen (eg tuberculin antigen) on antigen presenting cell comes in contact with naive CD-4 T cells ---> naive CD-4 T cells recognize these peptide antigen in association with MHC - II and differentiate in to T4-1 cells. o Induction of T4-I cells is of central impoance because the expression of delayed hypersensitivity largely depends on the cytokines secreted by T4-1 cells. 2. T-cell mediated cytotoxicity In this type of hypersensitivity, cytotoxic T cells kill virus infected or tumor cells.