How did doctors teach the immune system to fight cancer?

In 1891, an American surgeon named William Coley began injecting cancer patients with bacteria, after finding records of a patient whose tumor had vanished following an infection. He reported some striking recoveries, but his results were poorly controlled and documented, and the method faded. The idea survived, though. Nearly a century later, researchers found a real mechanism: the immune system can attack cancer cells, but some tumors exploit the very brakes T cells use to avoid attacking the body's own healthy tissue.

Those brakes are checkpoint molecules such as CTLA-4 and PD-1. Normally they stop T cells from overreacting against normal cells; cancer cells hijack them by displaying signals that tell nearby T cells to stand down. James Allison and Tasuku Honjo worked out how to block those brakes instead, freeing T cells to attack the tumor, and shared the 2018 Nobel Prize in Physiology or Medicine for it.

Checkpoint-blocking drugs are now approved treatments for several cancers, including melanoma and lung cancer. Releasing the brakes has a cost, though: many treated patients risk autoimmune-like side effects, because the brakes that tumors exploit are the same ones that normally stop T cells from attacking healthy tissue.