PDCD1 and head and neck squamous cell carcinoma: Monoclonal antibodies that target the programmed cell death protein-1/programmed cell death-ligand 1 (PD-1/PD-L1) and cytotoxic T lymphocyte-associated protein-4 (CTLA-4) pathways have brought about a permanent transformation in the treatment of various types of tumors, some of which were previously associated with a poor prognosis, including HNSCC [65,66].