Tlymphocytes mediate a number of cellular-immune reactions including cutaneous delayed hypersensitivity of the tuberculin or contact types, resistance to infection by intracellular facultative micro-organisms, host vs. graft and graft vs. host rejection phenomena, and tumour surveillance. These reactions result from complex cellular interactions between subpopulations of T lymphocytes and T lymphocytes and macrophages. The T cell may function directly as an effector cell or indirectly by orchestrating interactions between lymphocytes and macrophages. As a result of these activation signals, the macrophage may then develop into a more efficient phagocytic, ‘killer cell’ or in turn transmit signals that modulate T cell functions in either a positive or negative way.