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.