PDCD1 and head and neck squamous cell carcinoma: Immunotherapy with checkpoint blockade inhibitors, especially programmed cell death protein-1/programmed cell death ligand 1 (PD-1/PD-L1) blockades, have shown broad application for advanced cancer therapy, such as head and neck squamous cell carcinoma (HNSCC), lung cancer, melanoma, and esophageal cancer [4–7].