Agonist

An agonist can be a drug or chemical that binds to a molecular receptor and activates the receptor, typically causing it to generate an expected biological response. Although different agonists will bind with a particular receptor with varying degrees of affinity, some agonists may bind receptors irreversibly and certain other agonists may even be inverse agonists - agonists that bind a receptor but elicit responses that are opposite of those caused by ordinary agonists.