Illustrate how spare receptors contribute to a full response at low receptor occupancy.

• A. If spare receptors are present, a full response requires all receptors to be occupied.
• B. Spare receptors increase the concentration of ligand required for half-maximal response.
• C. A signal can be amplified through efficient signaling from a small fraction of receptors, yielding full effect before all receptors are occupied.
• D. Spare receptors only exist in in vitro systems.

Answer: (C)