The PD-1/PD-L1 pathway regulates immune tolerance in the tumor microenvironment.PD-1 and its ligands, PD-L1 or PD-L2, control T cell activation, proliferation, and cytotoxicity, impacting anti-tumor immune responses.PD-1 inhibits adaptive and innate immune responses, It has dual roles, beneficial in reducing ineffective immune responses and harmful by promoting tumor cell growth.