Comment by cowmoo728

Several varieties of T cells are very dangerous and like to murder other cells. In order to prevent them from going on a rampage, they have a switch called PD-1 that calms them down. This prevents various auto-immune diseases in healthy people.

Some varieties of cancer cells release a PD-1 ligand that turns off T cells when they get close to the cancer. So the cancer can "hide" from the immune system.

This monoclonal antibody blocks PD-1 on T cells, turning them into unstoppable murderers. The hope is that they preferentially murder the cancer cells. Wikipedia says that ~5% of patients get dangerous side effects from blocking T cell PD-1, probably because the unstoppable T cells attack healthy kidney or liver tissue. But for people with specific types of cancer, the hope is that turning the T cells loose will kill the cancer first.