CTLA4 and head and neck squamous cell carcinoma: Some researchers suggest that the lack of immunological control in SCCHN may also be driven by the expression of immune-inhibitory checkpoints, mainly the cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and the programmed cell death protein 1 (PD-1), which normally regulate the ongoing immune response to prevent damage to healthy tissues [56–58].