Apoptosis, or programmed cell death, is a form of cell death in which the cell activates an internal death program. It is characterized by nuclear DNA degradation, nuclear degeneration and condensation, and the phagocytosis of cell residua. Proliferating cells frequently undergo apoptosis, which is a natural process in development, and proliferating lymphocytes undergo high rates of apoptosis in development and during immune responses. Apoptosis contrasts with necrosis, death from without, which occurs in situations such as poisoning and anoxia.