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A hormone (from Greek horman - "to set in motion") is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms (including plants) produce hormones. The best known hormones are those produced by endocrine glands of vertebrate animals, but hormones are produced by nearly every organ system and tissue type in a human or animal body. Hormone molecules are secreted (released) directly into the bloodstream, other body fluids, or into adjacent tissues. 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. 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.

Hormone actions vary widely, but can include stimulation or inhibition of growth, induction or suppression of apoptosisIn biology, Apoptosis (pronounced ap-a-tow'-sis is one of the main types of programmed cell death (PCD). As such, it is a process of deliberate suicide by an unwanted cell in a multicellular organism. In contrast to necrosis, which is a form of cell death (programmed cell death), activation or inhibition of the immune systemThe immune system is any system present in an organism to prevent predation by biological agents. All living organisms have these protective measures, although they vary radically in scope and mechanism. In humans and domesticated animals, the immune syst, regulating metabolismMetabolism in the most general sense, is the ingestion and breakdown of complex compounds, coupled with the liberation of energy, and the consequent generation of waste products. It is major process of living organisms, and because this process can happen and preparation for a new activity (e.g. fighting, fleeing, mating) or phase of life (e.g. puberty, caring for offspring, menopause). In many cases, one hormone may regulate the production and release of other hormones. Many of the responses to hormone signals can be described as serving to regulateHomeostasis or homoeostasis is the property of an open system to regulate its internal environment so as to maintain a stable condition, by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. The term was co metabolic activity of an organ or tissue. Hormones also control the reproductive cycleThe reproduction of many organisms follows a cyclical pattern, known as the reproductive cycle which regulates the conception and birth of offspring. Many animals, including salmon, birds, deer and turtles synchronize their reproduction with the seasons i of virtually all multicellular organisms.