PDCD1 and head and neck cancer: The first anti-PD-1 inhibitory antibodies (Pembrolizumab and Nivolumab) received FDA approval in 2014 and since then different types of anti-PD-1/PD-L1 inhibitory antibodies are under development and/or have been approved for treatment of melanoma [27], metastatic non-small cell lung cancer [28], head and neck cancer [29], Hodgkin’s lymphoma [30] and gastroesophageal, bladder and urothelial cancers [31,32].