ABSTRACT
Higher organisms have developed elaborate mechanisms to rapidly and selectively eliminate unwanted cells by programmed cell death. Exposure or depletion of certain steroid hormones, incubation with noxious agents, loss of cell adhesion to the extracellular matrix, or dysregulated expression of oncogenes are only some of the conditions that can lead to cell death. A fine-tuned mechanism to regulate life and death of a cell is the interaction of surface receptors with their cognate ligands, which may either trigger a survival or, oppositely, an apoptogenic signal. Several receptors are able to transmit cytotoxic signals into the cytoplasm, but in most cases they have a wide range of other functions unrelated to cell death. The T and B cell receptors, CD2, CD4 and cytokine receptors, such as those for interferons, TGF-β or TNFrelated ligands, are examples of molecules that induce diverse signals resulting in cell activation, differentiation, proliferation, or induction of apoptosis. Whether the signals induced by a given receptor lead to cell activation or death is highly celltype specific and tightly regulated during differentiation of cells. For example, TNF receptors can exert costimulatory signals for proliferation of naive lymphocytes as well as can induce death signals required for deletion of activated lymphocytes.
