PDCD1 and neoplasm: In the tumor microenvironment, PD-L1 on the tumor cells surfaces combined with PD-1 on T cell surfaces can attenuate T cell-mediated immunosurveillance via various mechanisms, such as the induction of T cell non-response, failure, and even apoptosis; reducing the levels of cytokines such as tumor necrosis factor, interferon-γ, and interleukin-2; and inhibiting tumor infiltrating CD4-positive and CD8-positive T-cells (CD4+/CD8+ TILs), thus providing a way for cancer cells to evade the immune responses (106).