A process by which a cell breaks down and destroys old, damaged, or abnormal proteins and other substances in its cytoplasm (the fluid inside a cell). The breakdown products are then recycled for important cell functions, especially during periods of stress or starvation. Autophagy also helps destroy bacteria and viruses that cause infection and may prevent normal cells from becoming cancer cells. Once cancer has formed, autophagy may protect the cancer cells by providing extra nutrients to them or by keeping anticancer drugs or other substances from destroying them. Autophagy may also affect the body’s immune response against viruses, bacteria, and cancer cells.