Receptors – a complex, large molecules, located on the surface of cell membranes. Their main function is to transfer external impulses into the cell. In the role of these pulses are the various chemical substances contained in blood and interstitial fluid. Impact on the receptor to a substance starts a cascade of biochemical reactions inside cells, leading to the implementation of this cage of its functions. For example, insulin (A hormone produced by the pancreas), communicating with 'their' receptors located on fat cells, liver cells, muscle, and others, promotes the free entry of glucose molecules inside these cells. With a shortage of receptors to insulin, which is observed in some forms of diabetes in adults, glucose accumulates in the blood in excessive amounts.

Many drugs have the ability to either stimulate or block the work of the receptors. At the same time as any substance has its own well-defined receptor, and these drugs also act on specific receptors. Blocking a number of receptors leads to the fact that natural products of metabolism in the body is not able to contact them and cause any physiological response. Thus, H1-histamine receptor blockers to prevent these substances to cause the manifestation of allergic reactions and Conversely, drops in the nose, designed to combat the cold, by binding to the receptors for adrenaline, located in the vessels of the nasal mucosa, causing an effect similar to the action of the adrenaline, resulting in reduced edema and the development of discharge.