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Cell Mediated Immunity

An antigen may cause the production of antibodies (humoral immunity) or may give rise to sensitized lymphocytes (cell- mediated immunity). In cell-mediated immunity there is a direct 'reaction of lymphocytes with foreign material. Certain T-lymphocytes become sensitized to particular antigens. If they come across this antigen subsequently they react with it, and form substances known as 'lymphokines’. These substances affect macrophages and- other cells, resulting in delayed hypersensitivity skin reactions, e. g. the tuberculin reaction. Injection of the products of mycobacterium tuberculosis into the skin of a person previously infected with the bacterium results in the formation of a red indurate area at the site of injection in three or four days.