PD-1 (Programmed Cell Death Protein 1) is an immune inhibitory receptor, with its ligands including PD-L1 (Programmed Death-Ligand 1) and PD-L2. PD-1 is a single transmembrane structure primarily expressed on the surface of T cells, B cells, and some NK cells. PD-L1 is widely expressed on various cells, including tumor cells and immune cells. The binding of PD-1 to PD-L1 inhibits T cell activation and proliferation, regulates immune tolerance and self-immune responses, and prevents excessive activation of the immune system. In the tumor microenvironment, overexpression of PD-L1 can help tumor cells evade immune surveillance. Abnormal activation of the PD-1/PD-L1 signaling pathway is associated with various diseases, including cancer and autoimmune diseases, making it an important target for immune therapy.