PDCD1 and head and neck squamous cell carcinoma: The number of Food and Drug Administration (FDA)-approved agents blocking PD-L1/PD-1 axis is rapidly enlarging with indications for treatment of a broad spectrum of malignancies, ranging from classical Hodgkin lymphoma to head and neck squamous cell carcinoma (HNSCC), melanoma and urothelial cancers, both as monotherapy and in combination with other agents.