Conclusion
The concept of ‘receptive substances’ or ‘receptors’ arose from older notions of specific drugs and of elective affinities in the late nineteenth and early twentieth centuries within two different contexts: in immunology (with Paul Ehrlich) and in neurophysiology (with John Newport Langley). Both used the analogy of chemical binding between substances to explain the biological phenomena they studied. This chemical analogy implied the existence of specific substances or molecules in body cells that fixed biologically active substances, such as plant alkaloids, bacterial toxins, hormones and transmitter substances. The chemical character of the receptor concept led to controversy with those who favoured physical explanations of drug action. This conflict continued in the light of merely indirect evidence for about 60 years until the receptor concept led to tangible therapeutic consequences and receptors began to be identified with specific proteins embedded in cell membranes or inside the cells. The multitude of receptors that is now thought to exist provides a great challenge but also significant opportunities for the development of new specific treatments.