PD-1 (programmed cell death protein 1), also named CD279, is an inhibitory receptor expressed by all T cells during activation to control the body's immune response and is one of the most important immunity checkpoints. PD-1 inhibits T cell inflammatory activity and promotes self-tolerance, thereby regulating the immune system's response to human cells. Thus, PD-1 modulates T cell effector functions in a variety of physiological responses, including acute and chronic infections, cancer and autoimmunity, and immune homeostasis.