PDCD1 and neoplasm: When a T cell receptor (TCR) recognizes an antigen bound to the MHC, the co-receptor PD-1 on the surface of the immune cell, notably on cytotoxic T cells (CTL), binds to its ligand PD-L1 on different cells of the tumor microenvironment (TME), mainly on cancer cells’ surface, to trigger an inhibitory signaling to diminish T cells cytotoxic activity.