Thus, depending on the immune environment within different HNSCC tumour tissues, the PD-1/PD-L1 axis can be blocked at different levels: targeting PD-1 can block its binding to PD-L1/PD-L2, whereas targeting PD-L1 can inhibit its binding to PD-1/CD80. Here, PDCD1 is linked to head and neck squamous cell carcinoma.