Final answer:
The Fas ligand is a transmembrane protein on killer lymphocytes that activates Fas death receptors on a target cell, inducing apoptosis.
Step-by-step explanation:
In the extrinsic pathway to apoptosis, the Fas ligand is a transmembrane protein on the surface of killer lymphocytes that activate Fas death receptors on the surface of a target cell. Killer lymphocytes, such as cytotoxic T cells and natural killer (NK) cells, express the Fas ligand molecule that binds to the Fas molecule on a target cell, thereby inducing the target cell to undergo apoptosis. This mechanism is essential for the immune system to eliminate infected or cancerous cells and is part of the body's defense strategy against pathogens and diseases.