When T lymphocytes, or T cells, are activated by antigens, they also express immune checkpoints, such as PD-1. Other immune system cells express PD-1 ligands, such as PD-L1 and PD-L2, which interact with PD-1 receptors on T cells, dampening their activity and preventing the destruction of normal tissues by T cells (ie, autoimmune reactions). Tumor cells can also express PD-L1, which dampens the anti-tumor response of T cells. PD-1 inhibitors prevent the interaction of tumor-derived PD-L1 with PD-1, restoring the ability of T cells to attack and destroy tumor cells.