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Immunologic Defences Against Intracellular Organisms

Certain microorganisms, notably the Mycobacteria, and most pathogenic fungi and Protozoa are not killed after being engulfed. They not only survive but start to multiply intracellularly. These pathogens are inaccessible to many of the defence mechanisms useful against extracellular organisms. The infected cell must be lysed and the infectious agents contained therein must be killed. The major ­effector of both host cell lysis and microbial destruction is the activated macrophage.

Host defence against intracellular organisms is initiated mainly by the immune T-lymphocytes. The T-lymphocytes exposed to microbial antigens become sensitized. They undergo mitosis, and either diffe­rentiate into or stimulate the production of, antigen-specific effector T-cells. The immune T-effector cells release various lymphokines, e. g. monocyte chemotactic factor and the macrophage migration inhibition factor. These convert normal macrophages to activated macrophages.