Hormones are the chemical messengers of the endocrine system. Hormones are the signals which adjust the body's internal working, together with the nervous system. Every multicellular organism has hormones. The cells which react to a given hormone have special receptors for that hormone. When a hormone attaches to the receptor protein a mechanism for signalling is started. The cell or tissue that gets the message is called the 'target'. Hormones only act on cells which have the right receptors.
Many different kinds of cells can send a message. There are some kinds of cells whose main job is to make hormones. When many of these cells are together in one place, it is called a gland. Glands are groups of cells that make something and release it (put it outside the cell). Many glands make hormones. "Endocrine" means secreting directly into the blood. Most internal secretions are endocrine, from endocrine glands. The opposite word is "exocrine", which means secreting through a duct or tube. Some hormones are produced by exocrine glands, and some exocrine secretions go outside the body. Sweat glands and salivary glands are examples of exocrine glands whose products are released outside the body.

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