The role of Paul Ehrlich (1854–1915) in the development of the receptor concept is quite well-known [10], and I will therefore only briefly sketch it here (Figure 2). Ehrlich's side-chain-theory formed an important basis for his work on blood cells and on chemotherapy of infectious diseases such as sleeping-sickness and syphilis. In 1910, his research led to the introduction of Salvarsan, the famous ‘magic bullet’ against the germs of syphilis. Ehrlich's side-chain-theory was developed in the course of his studies into the staining of body cells and tissues, into the oxygen consumption of cells, and especially on the interaction between bacterial toxins and the so-called anti-toxins or antibodies formed by the body. In 1897 Ehrlich published for the first time a full account of his side-chain-theory of anti-toxin formation. The large ‘molecule’ of the cell protoplasm was supposed to have certain side-chains that were able to bind chemically the toxins produced by the bacteria. The thus occupied side-chains became unable to fulfil their usual functions in nutrition and oxygen consumption, forcing the cell to produce more side-chains. A surplus of side-chains was released into the blood stream where they bound as anti-toxins or antibodies to the bacterial toxins—forming thus the basis of immunity (Figure 3). In 1900 Ehrlich replaced the term side-chain (or Seitenkette) with the term Receptor [11].