Vaughan, in his delightful "Memories,"1 quotes Metchnikoff, the father of the phagocytic doctrine, as saying twenty years ago, "If a German does not believe in Ehrlich's side chain theory, he is damned." The statement well expresses the strong hold that Ehrlich's view had on the German scientific mind of that day. True, Bordet and his pupils refused to be counted among the "believers," but, with the exception of this group, the theory has had practically universal acceptance. To this day few textbooks in bacteriology fail to present colorful pictures of the toxin molecule uniting with the cell receptor antibody, fitting into each other as a "key fits into a lock."
Ehrlich, in formulating his theory, went for analogy to the realm of nutrition. He conceived the body cell as a complex mass of protoplasm, consisting of a central and relatively stable nucleus, surrounded by innumerable atomic groups or side chains,