PDCD1 and head and neck cancer: Anti-programmed cell death 1 (anti-PD-1) or programmed death-ligand 1 (PD-L1) blocking antibodies have shown objective responses in a variety of solid tumors, including melanoma, lung cancer, prostate cancer, breast cancer, ovarian cancer, head and neck cancer, and a subset of colorectal cancers [53,54,55,56,57,58].