"In cancerous cells, checkpoint proteins, such as PD-L1 on tumor cells and PD-1 on T cells, help keep immune responses in check. The binding of PD-L1 to PD-1 prevents T cells from ""seeing"" the tumor cell, allowing the cancer to grow unchecked (left panel). Blocking the PD-1/PD-L1 interaction with an immune checkpoint inhibitor (anti-PD-L1 or anti-PD-1) unshields the tumor cell so T cells can kill it (right panel). Source: National Cancer Institute. Printed with permission © 2015 Terese Winslow LLC, U.S. Govt. has certain rights."
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