PD-1 (programmed cell death protein 1) is an important immunosuppressive molecule. As a member of the CD28 superfamily, it was originally cloned from an apoptotic mouse T-cell hybridoma 2B4.11. The immune regulation targeting PD-1 is of great significance in combating tumors, infections, autoimmune diseases, and organ transplant survival. Its ligand PD-L1 can also serve as a target, and corresponding antibodies can also play the same role. PD-1 (programmed cell death protein 1), also known as CD279, or PDCD1, is an important immunosuppressive molecule. As a member of the CD28 superfamily, it was originally cloned from an apoptotic mouse T-cell hybridoma 2B4.11. The immune regulation targeting PD-1 is of great significance in combating tumors, infections, autoimmune diseases, and organ transplant survival. Its ligand PD-L1 can also serve as a target, and corresponding antibodies can also play the same role.