PDCD1 and neoplasm: PD–1/PD–L1 inhibitors and anti–CTLA–4 antibodies have distinct mechanisms of action, as the PD–1/PD–L1 pathway regulates inflammatory reactions in tumor microenvironments and peripheral tissues, and its activation occurs later during the immune response, while CTLA–4 is induced in T cells earlier in response to antigens [124].