PDCD1 is an immunoreceptor that belongs to the immunoglobulin (Ig) superfamily, also known as Programmed Cell Death 1. It is expressed on activated T cell, B cells, myeloid cells and a subset of thymocytes. PDCD1 interacts with expressed PDL1 and PDL2 to inhibit the proliferation and activation of T cells and B cells. Both PDL1 and PDL2 are expressed in a wide variety of normal tissues including heart, lung, placenta, spleen and kidney. PDCD1 might be involved in the regulation of peripheral tolerance and may help prevent autoimmune diseases