Final answer:
Toll-like receptors (TLRs) require the extracellular proteins MD2, CD14, and LBP to initiate the production of cytokines, which are signaling molecules in immune responses.
Step-by-step explanation:
The extracellular proteins that Toll-like receptors (TLRs) require to start the cytokine-producing protein cascade are MD2, CD14, and LBP. TLRs are proteins on the cell membrane of immune cells that recognize molecules characteristic of invaders. MD2, CD14, and LBP help TLRs recognize these molecules and initiate the production of cytokines, which are signaling molecules that play a crucial role in immune responses.