in biology, a mechanism that allows cells to self-destruct when stimulated by the appropriate trigger. Apoptosis is initiated for various reasons, such as when a cell is no longer needed within the body or when it becomes a threat to the health of the organism. Apoptosis is necessary in embryonic development as well as in the daily maintenance of a mature organism. The aberrant inhibition or initiation of apoptosis contributes to many disease processes, including cancer.

Embryologists in the early 20th century were familiar with the process of programmed cell death. They observed that as an embryo develops, many of its cells are sacrificed to create the final form of the organism. Not until 1972, however, did researchers John F.R. Kerr, Andrew H. Wyllie, and Alastair Currie recognize the broader significance of this mechanism. They coined the term apoptosis from the Greek word meaning “falling off,” as leaves do in autumn, to describe this natural, timely death of cells. Apoptosis is distinguished from an alternate form of cell death, called necrosis, that results from injury.

Apoptosis is a normal physiological process that offsets cell proliferation. It is a genetically programmed event that can be set in motion by a variety of internal or external stimuli. A signal activates genes in the cell's suicide pathway which encode the proteins that destroy the cell's structural proteins and genetic material. A number of morphological changes occur in the apoptotic cell—e.g., the cell begins to shrivel and pull away from other cells, bubblelike formations appear on its surface, and chromatin (chromosomal DNA and protein) in the cell's nucleus condenses. The cell then either is consumed by other cells or breaks up into smaller pieces that are engulfed by scavenger cells. Cells in virtually all tissues may be sacrificed through apoptosis for the good of the organism. For example, the cells of the uterine wall undergo programmed death in the monthly menstrual cycle.

Various phenomena can disturb the regulation of the cell death pathway, causing too many or too few cells to die. Many types of disease result from such disruptions. If, for example, a mutation occurs in a gene that induces apoptosis, such as the tumour-suppressor gene p53, the cell that harbours the gene may fail to respond to the cue to die. As a result the cell may proliferate uncontrollably and form a cancerous tumour. In other cases a virus may interfere with the regulation of apoptosis, inducing healthy cells to die. This mechanism is believed to play a role in AIDS, the disease in which infection with the HIV virus results in the destruction of healthy white blood cells called T lymphocytes.