ABSTRACT
The concept of specific receptors on the cell surface was outlined by Langley in 1905, but was not given substance until the 1970s, when biochemical techniques were able to extract and purify receptor proteins and allow their characterization ex vivo. Previously, receptors were classified in vivo or in isolated tissues on the basis of their relative responses to series of agonists and antagonists. Much of what we know about the basic properties of receptors and about agonists and antagonists comes from the work of Gaddum, Clark, Schild, and others during that period. Classification of receptors during this period was based solely upon the pharmacology of tissue responses.
