Agonist

An agonist is a type of chemical known as a ligand. It binds to a receptor. It activates (switches on) the receptor to produce a response.

An agonist causes an action. There are also antagonists which block the action of the agonists, and there are inverse agonists which cause an action opposite to that of the agonist.

This is typical of the systems which regulate the body. Being able to control activity is the basis of homeostasis.