Up-regulation refers to which of the following in receptor biology?

• A. The decrease in number of receptors
• B. No change in receptor numbers
• C. The increase in number of receptors
• D. Desensitization

Answer: (C)

Up-regulation is the increase in the number of receptors on the cell surface, which raises sensitivity to the ligand. Cells do this to boost signaling when ligand levels are low or signaling is blocked, making it easier for any available ligand to bind and produce a response. This contrasts with down-regulation, where receptor numbers fall and responsiveness decreases; no change would mean receptor density stays the same; desensitization refers to a diminished response despite receptor presence, often from signaling changes rather than more receptors being made. So increasing receptor numbers best describes up-regulation.