PDCD1 and head and neck squamous cell carcinoma: Immune-checkpoint inhibitors (ICI), monoclonal antibodies targeting immune checkpoints [programmed cell death protein 1 (PD1), programmed cell death protein 1 ligand (PD-L1), cytotoxic T-lymphocyte-associated protein-4 (CTLA-4)], have changed the treatment landscape of locally advanced and R/M HNSCC.