Final answer:
Both agonists and antagonists can bind to a receptor (affinity), but only agonists can activate the receptor (efficacy). Agonists mimic neurotransmitters and initiate a response, whereas antagonists block the receptor and prevent activation.
Step-by-step explanation:
Both agonists and antagonists have affinity but only agonists have efficacy.
Agonists are chemicals that mimic a neurotransmitter at the receptor site and can initiate a physiological response. They have both affinity, meaning they can bind to the receptor, and efficacy, which is the ability to activate the receptor after binding. In contrast, antagonists also have affinity for the receptor because they can bind to it, but they lack efficacy and therefore do not activate the receptor and can block the activity of other substances that have efficacy.
For example, in treating Parkinson's disease, which involves low levels of dopamine, doctors may prescribe dopamine agonists. These drugs have the affinity to bind to dopamine receptors, and because they have efficacy, they can mimic the effects of dopamine and activate the receptors.