HAVCR2 and head and neck squamous cell carcinoma: Ligands for inhibitory receptors on circulating lymphocytes, such as CTLA-4, PD-1, (PD-L1 is the ligand for PD-1) and T cell immunoglobulin mucin-3 (TIM-3), are overexpressed by HNSCC tumor cells and are associated with worse survival [37,38,39].