Neuropeptides are regulatory molecules produced by neural and non-neural tissues. They are engaged in multiple forms of receptor-mediated intercellular communication,ranging from hormonal (blood-borne) to close-range (synapse-like) signalling. In their neurohormonal capacity, these peptides convey directives from the nervous to the endocrine system, as well as to terminal target tissues. As participants in interneuronal chemical communication, neuropeptides may act as neurotransmitters, or as modulators of signals exchanged between synaptically linked neurons. Outside the realms of neuroendocrinology and neurobiology, peptidergic messenger substances carry out a variety of functions, including autoregulatory and integrative activities in the immune system