PD-1 is a protein found on T cells that helps keep the body’s immune responses in check. When PD-1 is bound to another protein called PD-L1, it helps keep T cells from killing other cells, including cancer cells.
Some anticancer drugs, called immune checkpoint inhibitors, are used to block PD-1. When this protein is blocked, the “brakes” on the immune system are released and the ability of T cells to kill cancer cells is increased.
(Source: National Cancer Institute)