Programmed cell death in immune system, illustration. The protein PD-1 (programmed cell death protein 1, red) is seen extending from the surface membrane of a T-cell (across bottom), interacting with the ligand protein PD-L1 (orange) from an antigen presenting cell (APC). Although the T-cell has been activated through the interaction at left of a T-cell receptor (blue) and a MHC protein (purple), PD-1 regulates and reduces this activation. It acts to slow down the effects of T-cells.