PDCD1 and neoplasm: The activity of TILs can be impeded by antigen-presenting cells or tumor cells through immune checkpoints such as programmed cell death protein 1 (PD-1) or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) expressed on the T-cell surface, which can be blocked by checkpoint inhibitors such as ipilimumab (anti-CTLA-4), nivolumab and pembrolizumab (both anti-PD-1)27.