The body consists of cells which communicate with each other via signaling molecules to govern and determine a variety of physiological functions in the body. The action and effect of these signaling molecules is mediated by ‘receptors’ which are located on the surface of (plasma membrane), or inside a cell. A molecule that ‘binds’ with a receptor is known as a ligand and can be protein such as a hormone or drug and the role of the ligand is to activate, or inactivate a particular biological activity.

In some ways receptors can be likened to switches with on and off positions, and which in turn affect the cell’s internal functioning. The action or inaction of receptors is determined by the type of ligand with which they bind, i.e. receptors are activated when an agonising ligand, an ‘agonist’ binds and left inactive when an ‘antagonist’ in some way prevents the agonist from binding. The specific biological action is dependent upon which particular ligand binds with a receptor.

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