Immune checkpoint inhibition, illustration. Immune checkpoints are regulators of the immune system. Here, antibodies are shown blocking PD-L1 (programmed cell death 1 ligand 1, red) on the surface of a cancer cell and the immune checkpoints PD-1 (programmed cell death 1, blue) and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4, violet) on a T-cell. If PD-1 and CTLA-4 proteins were activated, it would inhibit T-cells from killing the cancer cells.