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Programmed cell death protein 1, also known as PD-1 and CD279 （cluster of differentiation 279）, is a protein on the surface of T and B cells that has a role in regulating the immune system's response to the cells of the human body by down-regulating the immune system and promoting self-tolerance by suppressing T cell inflammatory activity. This prevents autoimmune diseases, but it can also prevent the immune system from killing cancer cells. PD-1 is an immune checkpoint and guards against autoimmunity through two mechanisms. First, it promotes apoptosis （programmed cell death） of antigen-specific T-cells in lymph nodes. Second, it reduces apoptosis in regulatory T cells （anti-inflammatory, suppressive T cells）.