Why do damaged cells sometimes multiply instead of cleaning themselves up?

Cells have several mechanisms for cleaning themselves up when something goes wrong: they can die in an orderly fashion (apoptosis), recycle damaged components (autophagy), or disappear through a more explosive inflammatory death (pyroptosis). In cancer, all of these mechanisms are hijacked or disabled in different ways.

Apoptosis, the best-known form of programmed cell death, is often blocked in cancer cells. Cancer cells produce proteins that remove degradation signals from damaged proteins, causing the 'self-destruct button' to stop working. The consequence is doubly problematic: not only do the cells survive longer, they also become much harder to kill with chemotherapy. Chemotherapy works largely by triggering apoptosis, and if that pathway is blocked, the treatment misses its target.

Autophagy, the cell's recycling mechanism, behaves in an even more complex way. In early stages it suppresses tumour formation, but in advanced cancer its role reverses: the tumour uses autophagy as a survival strategy instead. Which factors exactly flip the switch is not yet fully understood.

There is also a paradox in cell death itself: in aggressive tumours, large numbers of cells actually do die, yet this tends to advance the cancer rather than slow it down. The dead cancer cells activate the immune system in a way that generates repair and growth signals in the tumour's surroundings. Surviving cancer cells benefit from this. Finally, pyroptosis, a third form of cell death in which an inflammatory response erupts, plays an unclear dual role in tumours: sometimes it kills tumour cells, and sometimes it actually helps the tumour evade the immune system. This depends on context, and research into it is still in the preclinical phase.