PDCD1 and head and neck squamous cell carcinoma: Examples of well-known immuno-mechanisms include the CTLA-4/CD152 pathway in melanoma [66], the programmed cell death protein 1 (PD-1/CD279) pathway in various types of cancer such as melanoma, RCC, HNSCC, NSCLC, ovarian cancer, Merkel cell carcinoma, B-cell lymphoma, follicular lymphoma, and urothelial carcinoma [68,69], and the human epidermal growth factor receptor (EGFR) 2-directed monoclonal antibody in HNSCC and CRC [70].