Illustration of the binding of the surface protein PD-L1 (programmed cell death 1 ligand 1, yellow) to PD-1 (red). Programmed cell death 1 ligand 1 is involved in the inhibition of the immune response together with the receptor PD-1. PD-1 is found on the surface of T cells and interacts with the ligand protein PD-L1 which is found on the surface of regulatory cells, such as the cells that present antigens from invaders.