<qa>  What is the name of the immune system that blocks PD-1?Programmed cell death protein 1, also known as PD-1, functions as an immune checkpoint, down-regulating the immune system by preventing the activation of T-cells, which in turn reduces autoimmunity and promotes self-tolerance. A new class of immunotherapy drugs that block PD-1, the PD-1 inhibitors, activate the immune system to attack tumors and are therefore used with varying success to treat some types of cancer. 