Antibody blocking programmed cell death, illustration. Antibody (yellow) blocking PD-1 (programmed cell death protein 1, red) to prevent it interacting with a surface protein called PD-L1 (orange). Cancer cells (membrane across top) express PD-L1 proteins on their surface to trick the immune system. The interaction of PD-L1 with PD-1 of T-cells (membrane across bottom) affects the T-cell functionality. The antibody is blocking this interaction. For the interaction without blocking, see image F023/0921.