PDCD1 and head and neck squamous cell carcinoma: The PD-1/PD-L1 pathway is one such immune regulatory nexus that has been shown to have a promising anti-tumor role in the non-small cell lung cancer, head and neck squamous cell carcinoma, cervical cancer, renal cancer, gastric cancer, chronic Hodgkin’s lymphoma, hepatocellular carcinoma, urothelial cancer, and melanoma (12).