Hormone

A hormone  [Gk. hormao, instigate] is defined as an organic chemical produced by one set of cells that affects a different set. A hormone usually travels through the circulatory system to its target organ. It is important to stress that a hormone does not seek out a particular organ; to the contrary, the organ is awaiting the arrival of the hormone. Cells that can react to a hormone have specific receptors, which combine with the hormone in a lock-and-key manner. Therefore, certain cells respond to one hormone and not to another, depending on their receptors.