PDCD1 and cancer: Immune escape from PD-L1/PD-1-targeted therapy includes the downregulation of the major histocompatibility complex in cancer cells, the minimal activation of cancer-specific T cells, the poor infiltration of T cells into tumors, the lack of strong cancer antigens or epitopes recognized by T cells, and the presence of immunosuppressive factors and cells in the tumor microenvironment [172].