Hormones are chemical messengers that are secreted by the endocrine glands into the blood. They produce a specific effect on the activity of cells that are remotely located from their point of origin. Hormones are transported via the bloodstream to reach specific cells, called target cells, in other tissues. Target cells have special receptors on their outer membranes that allow the individual hormones to bind to the cell. The hormones and receptors fit together much like a lock and key.