PDCD1 and cancer: Naked antibodies that lack any type of drug conjugation work by either: (1) stimulating the immune system by binding to an antigen present on a cancer cell (alemtuzumab); (2) boosting the immune response via interaction with immune-checkpoint proteins (CTLA-4 inhibitors (ipilimumab)/PD-1 inhibitors (pembrolizumab); or (3) blocking growth factor receptors on cancer cells (trastuzumab).