PDCD1 and neoplasm: PD-L1 on the surface of tumors can bind to PD-1 on the surface of activated T lymphocytes, thereby reducing the activity of T lymphocytes and inhibiting lymphocyte proliferation through the PD-1/PD-L1 pathway, thus suppressing the role of T lymphocytes in the local tumor microenvironment, reducing the immune killing function of tumors, and making tumors prone to immune escape (14).