https://www.cancer.gov/about-cancer/treatment...inhibitors
Checkpoint proteins, such as PD-L1 on a tumor cell and PD-1 on a T cell ( limits the body's natural immune response.) The binding of PD-L1 on a tumor cell to PD-1 on a T cell keeps the T cells from killing tumor cells in the body (left panel).
Blocking the binding of PD-L1 to PD-1 with an immune checkpoint inhibitor (anti-PD-L1 or anti-PD-1) allows the T cells to kill tumor cells (right panel)