PDCD1 and head and neck squamous cell carcinoma: Some of the immune escape mechanisms of HNSCC include the recruitment of inhibitory cell populations such as Treg, MDSC, TAM, and CAF; perturbation by ICs such as PD-1 and CTLA-4, leading to T cell exhaustion; dysregulation of pro-proliferative cytokines such as TGF-beta, IL-6, and IL-10, as well as signaling pathways such as STAT-3; and increased physical barriers that hinder the infiltration of effector T cells and other immune cells [129,130,131].