Agonist

<p>An&nbsp;<b >agonist</b>&nbsp;is a chemical that binds to a&nbsp;<a href="http://en.wikipedia.org/wiki/Receptor_(biochemistry)" title="Receptor (biochemistry)" >receptor</a>&nbsp;of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance.&nbsp;</p>

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Whereas an agonist causes an action, an&nbsp;<a href="http://en.wikipedia.org/wiki/Receptor_antagonist" title="Receptor antagonist" >antagonist</a>&nbsp;blocks the action of the agonist and an&nbsp;<a href="http://en.wikipedia.org/wiki/Inverse_agonist" title="Inverse agonist" >inverse agonist</a>&nbsp;causes an action opposite to that of the agonist.</p>