A hormone is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms produce hormones (including plants - see phytohormone).
The function of hormones is to serve as a signal to the target cells; the action of hormones is determined by the pattern of secretion and the signal transduction of the receiving tissue The best-known animal hormones are those produced by endocrine glands of vertebrate animals, but hormones are produced by nearly every organ system and tissue type in an animal body.
Hormone molecules are secreted (released) directly into the bloodstream; some hormones, called ectohormones, are not secreted into the blood stream, they move by circulation or diffusion to their target cells, which may be nearby cells (paracrine action) in the same tissue or cells of a distant organ of the body.