Agonist

thumb|400 px|Dose response curves of a full agonist, partial agonist, neutral antagonist, and inverse agonist An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agonist, while an inverse agonist causes an action opposite to that of the agonist.