They are defined as any substance that is produced in one part of the body and causes an action somewhere else, but beyond that they are not easy to characterize. They come in different sizes, have different chemistries, go to different places, have different effects when they get there. Some are proteins, some are steroids, some are from a group called amines. They are linked by their purpose, not their chemistry. Our understanding of them is far from complete, and much of what we do know is surprisingly recent. John Wass, professor
