Hormones are chemical messengers released by cells to modulate signal transduction in other cell populations. Hormones can be released directly into the bloodstream (endocrine), enter the blood via secretion through a specific duct (exocrine), or simply diffuse from one cell to another (paracrine). Hormonal signaling impacts almost every aspect of physiology, including metabolism, reproduction, immune function, growth, and mood. Specific hormone molecules exert their signaling effects through specific receptors expressed on target cells. However, it is known that some hormones will interact promiscuously with multiple types of hormone receptors. R&D Systems offers a wide range of hormone products to study eicosanoids, protein/peptide hormones, steroid hormones and the Renin-Angiotensin System (RAS).