Cell mediated immunity is-

Ans. is 'd' i.e., IV Cell mediated (Type) hypersensitivityo It is initiated by activated T-lymphocytes.o It includesDelayed type hypersensitivity - mediated by CD-4 T cells.T-cell mediated cytotoxicity - mediated by CDS- T cells.1. Delayed type hypersensitivityo 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,o Delayed hypersensitivity is a major mechanism of defence against various intracellular pathogens, including mycobacteria, fungi and certain parasites.Pathophysiologyo Delayed hypersensitivity is characterized by formation of granuloma - a localized mass that contains activated macrophages (epitheloid} ceils surrounded by collor of lymphocytes and giant cells,o It is also cal led granulomatous Inflammation.o The cellular events starts 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-l cells.o Induction of T4-l cellsis ofcentral importance because the expression of delayed hypersensitivity largely depends on the cytokines secreted by T4-l cells.2 T-cell mediated cytotoxicityo In this type of hypersensitivity, cytotoxic T cells kill virus infected or tumor cells.