In the body, there is a type of white blood cell that watches out for signs of infection or disease and can also assault cancer cells. These white blood cells are called T cells. On the surface of these T cells are certain proteins known as programmed cell death receptors (PD-1). So that T cells don’t destroy healthy cells, they use these surface proteins, or PD-1, to bind to and communicate with PD-L1 and PD-L2, proteins that permit cells to escape attack from the body’s immune system.