Pd-l1 is a ligand protein that is expressed on the surface of a healthy cell.
Pd-l1 binds with Pd-1 checkpoint ligand on the surface of T-CellS.
The binding of these ligands is called the checkpoint pathway.
Checkpoint pathway purpose
When a Healthy cell is approached by a T Cell that is primed to attack, the T Cell has a checkpoint switch called Pd-1 (ligand) which can down regulate (stop) an accidental attack on healthy cells.
A healthy cell will express the Pd-l1 ligand on its surface when confronted by a T Cell and bind with the T Cells Pd-1 – a handshake between friends.