Which option defines an antagonist?

• A. A molecule that binds to a receptor and produces a response
• B. A molecule that binds to a receptor and decreases the effect of an agonist; does not produce any response by itself
• C. A molecule that binds to a receptor and increases the receptor activity
• D. A molecule that blocks receptor signaling by internalizing the receptor

Answer: (C)

An antagonist is a molecule that binds to a receptor but does not activate it, and by occupying the receptor it blocks or reduces the effect of other molecules that would activate the receptor (agonists). It has no intrinsic activity on its own. This is why the description that emphasizes binding to a receptor, producing no response by itself, and decreasing the effect of an agonist best defines an antagonist. In contrast, an agonist both binds and activates the receptor to produce a response, and while receptor internalization can be a downstream effect of signaling blockade, it’s not the defining feature of an antagonist.