ABSTRACT
This chapter explores the fundamental concepts of receptor theory. It presents how the two main types of receptor work, how endogenous and exogenous chemical messengers may activate or block receptors, and how this chemical message at the cell surface is changed to a quite different signal within the cell, sometimes with great amplification, altering cell function. One important feature of receptors with enzyme system effectors is the capacity for great amplification of the original chemical signal. Many of the drugs mimic endogenous chemical signals by binding as ligands at receptors on cell membranes. Some are agonists; others are antagonists, depending on their effects at the receptor. The relevant features of intercellular signalling are as follows: receptor recognises its ligand and binds it with a reversible chemical bond. The individual cell exists only in the context of its society, and this has probably been the case ever since the first multicellular organisms appeared some billion years ago.
