How does an antagonist function at a receptor site?

An antagonist functions at a receptor site by binding to the receptor and blocking its activation. This means that the antagonist occupies the receptor but does not initiate a response; instead, it prevents other substances, such as agonists or endogenous ligands, from binding and triggering the typical effects that would occur if the receptor were activated. By doing so, antagonists can effectively inhibit the physiological actions that would normally result from receptor activation.

In contrast to other options, an antagonist does not activate the receptor to produce effects, nor does it increase the receptor's affinity for ligands or enhance endogenous responses. The role of an antagonist is purely to inhibit or prevent the biological activity that would occur upon activation of that receptor. This makes antagonists vital in various therapeutic contexts where reducing or blocking certain physiological effects is desired.