Apoptosis is a form of programmed cell death in which cells play an active role in their own death, ensuring that they die in a controlled and regulated manner with little disruption to the surrounding cells. Apoptosis allows unwanted cells (for example, damaged or virus-infected cells) to be efficiently removed from the body. It plays important roles in a variety of biological processes, including embryo development, the immune system, and cancer prevention.

Cells undergo apoptosis in response to a wide variety of stimuli, both internal and external. Internal signals include DNA damage, cytoplasmic stress, and nutrient deprivation. By contrast, external signals can induce apoptosis by interacting with a class of cell surface receptors known as death receptors. The death receptors bind a variety of ligands known collectively as the death-inducing ligands, which can exist either as soluble, secreted proteins or as membrane-bound proteins on the surface of cells such as T lymphocytes. The binding of these ligands to the death receptors allows the apoptotic signal to be transmitted to the cell, triggering the onset of apoptosis.