The most fundamental question facing each and every cell within an org- ism is to survive or to die. Cell death is required for normal function; some estimates suggest that as many as one million cells undergo cell death every second in the adult human body. Almost all cells undergoing physiological, or programmed, cell death, independent of cell type, manifest a stereotypic p- tern of morphological changes termed apoptosis. Typically, apoptotic cells d- play shrinkage, membrane blebbing, chromatin condensation, and nuclear fragmentation. The integrity of the cell membrane is not lost during apoptosis and so avoids eliciting the inflammatory response that would have been caused by the spillage of the cell's contents. This is quite in contrast to the loss of cell contents typical of necrosis. The caspases, the family of intracellular cysteine proteases associated with apoptosis, are responsible for the stereotypical m- phological changes. Caspases cleave various substrate proteins that act on DNA fragmentation, nuclear envelope integrity, the cytoskeleton, and cell volume regulation. Apoptotic cells are cleared in vivo by the process of phagocytosis, in which specific "phagocytes" move to the site of apoptosis, engulf the dying cells and digest them. Apoptosis has a central role in many physiological processes, for example, in the immune system. Autoreactive cells are deleted via apoptosis to prevent autoimmunity. At the end of an immune response, activated lymphocytes are removed to maintain homeostasis within the immune system.
