PDCD1 and cancer: ICIs are a novel category of drugs that are essentially humanized monoclonal antibodies, which activate T cells and relieve the immune system to recognize and assault cancer cells by targeting cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) (CD152), programmed cell death protein 1 (PD-1), or programmed cell death ligand 1 (PD-L1).