Hitting the brakes: how PD-1 stops T cells from fighting

Pseudo-colored scanning electron micrograph of an oral squamous cancer cell (white) being attacked by two cytotoxic T cells (red), part of a natural immune response.

PD-1 is part of a group of proteins that helps promote balance during an immune response. Together, these are known as immune checkpoints. These “break” proteins help decrease the function of our T cell soldiers, which is good as we don’t want our great fighters killing when there is nothing to kill.