Studies have shown that PD-1 signaling is one of the main factors in human cancer immune escape. PD-L1 is a PD-1 receptor located on the surface of normal cells and cancer cells. When PD-1 binds to PD-L1, it helps prevent T cells from killing other cells, including cancer cells. Monoclonal antibodies that target PD-1 or PD-L1 block this binding and enhance the immune response of T cells to cancer cells.