[Acquired resistance to facultative intracellular bacteria: as to the identity of the cell mediating protection and delayed hypersensitivity (author's transl)].

Acquired resistance to facultative intracellular bacteria depends on two interacting classes of cells: antigen specific T lymphocytes and mononuclear phagocytes. After specific interaction with antigen, T lymphocytes are capable of activating mononuclear phagocytes to form granulomas and to acquire enhanced bacteriocidal capacity. In general, protection is paralleled by delayed hypersensitivity to bacterial antigens. Although Robert Koch had already postulated that protection against Mycobacterium tuberculosis and delayed hypersensitivity to tuberculin in principle depend on an identical mechanism, this question has been unresolved thus far. Recently it has become possible (a) to select heterogeneous T cell subpopulations by serologic methods and (b) to clone and propagate homogeneous T cell lines in a biologically active form. Applying these techniques, we could show that a single T cell population specific for the intracellular bacterium, Listeria monocytogenes, is capable of mediating both antibacterial protection and delayed hypersensitivity. These data show that both functions in principle depend on an identical mechanism thus resolving the problem in Robert Koch's original sense.