Final answer:
The initiator caspase-activated Bid when the Fas death receptor activates the extrinsic pathway to apoptosis, with caspase-8 usually being the initiating caspase implicated in this pathway.
Step-by-step explanation:
When the Fas death receptor activates the extrinsic apoptosis pathway, the initiator caspase, usually caspase-8, is activated, which in turn can cleave and activate Bid, a pro-apoptotic Bcl-2 family member. This process is crucial in apoptosis, also known as programmed cell death, and the Fas receptor is a significant component of this pathway. The Fas ligand is a molecule expressed in cytotoxic T cells and NK cells, and it binds to the Fas molecule on a target cell, leading to the target cell's apoptosis. This is part of the immune system's response to remove cells that are abnormal or no longer needed.