PD-1 is programmed death receptor 1, a member of the CD28/cytotoxic T lymphocyte-associated protein 4 receptor superfamily, and expresses mainly immune system T cell immune properties, but also in B cells, natural killer cells and myeloid-derived cells, all of which may appear to be immune enhancingly regulated. Binding to its ligands PD-L1 and PD-L2 mediates immunosuppressive signaling, which has the effect of suppressing T lymphocyte activity and downregulating the immune response.PD-L1/PD-1 signaling can act as a dual inhibitor of T cell activation in the initial environment of the immune response and at organ tissue sites, causing and maintaining immune tolerance.