ii6 Immunity 



in their resistance to diphtheria toxin, and that the serum of the 

 resisting horses contains something that destroys or neutralizes the 

 toxin in vitro, as well as exerts a protective influence upon animals 

 into which it is injected. This substance exerts no inimical action 

 upon the diphtheria baciUi, beyond what a normal serum would do, 

 therefore cannot be alexin, but must be antitoxin. Abel* found that 

 the blood of healthy men occasionally contained some substance 

 capable of neutralizing diphtheria toxin; Stern found one normal 

 serum capable of protecting against typhoid infection and Metch- 

 nikofl! one that protected against cholera infection. Fischel 

 and Wunschheimf found newly born babies immune against diph- 

 theria, presumably because of the presence of a smaU quantity of 

 demonstrable protective substance in the blood. These are, how? 

 ever, peculiar and exceptional cases. 



The most suggestive and fascinating explanation of immunity is 

 that of Paul Ehrlich, known as the " Seitenkettentheorie," or the 

 "Lateral-Chain Theory of Immunity. "t 



It was the outgrowth of philosophic speculation concerning the 

 mechanism of cell-nutrition, of observation of the behavior of certain 

 anilin dyes when brought into contact with living cells, of studies of 

 the composition of diphtheria toxin, and the application of Carl 

 Weigert's law of regeneration applied to the requirements of cell 

 life. Like all great theories it has been the subject of much contro- 

 versy, and not a few of its adversaries believe that they have com- 

 pletely disproved it. Whether it be adequate to meet the require- 

 ments of increasing knowledge of immunity the future must decide. 

 Of its present usefulness there can be no doubt. It has been to tlie 

 investigation of the problems of immunity, like the theory of evolu- 

 tion has been to the biological sciences, a most convenient and sug- 

 gestive method of reasoning and deduction. 



The theory begins with the supposition that all living cells possess 

 certain functions that are individual, fundamental and indispensable 

 and that such cells as are components of multicellular organisms 



* "Centralbl. f. Bakt.," etc., .1895, xvii, p. 36. 



t "Zeitschr. flir Heilkunde," 189s, XVI, p. 429-482. 



t The writings of Ehrlich and his associates are so numerous and scattered, 

 and often so fragmentary, that instead of referring to the literatture accordiag to 

 the method adopted in other parts of this work, the reader who desires to consult 

 the original articles can best do so by making use of the following: Ehrlich, 

 "Die Werthbemessung des Diphtheric Heilserums," Klinisches Jahrbuch, 1897; 

 Ehrlich, "Die Konstitution des Diphtheriegiftes," Deutsche med. Woch., 1898; 

 Gesamnielte Arbeiten zur Immunitatsforschung," August Hirschwald, Berlin, 

 1904— this work contains the collected papers of Ehrlich and his associates; 

 Aschofif, "Ehrlich's Seitenkettentheorie und ihre Anwendung auf die Kunst- 

 lichen Immunusirungs-prozesse," Jena, 1902, and the chapter upon "Wirkung 

 und Entstehung der Aktiven Stoffe im Serum noch der Seitenkettentheorie," 

 by Ehrhch and Morgenroth in KoUe and Wassermann's "Handbuch der Patho- 

 gene Mikroorganismen," Jena, 1904, Gustav Fischer. Readers unacquainted 

 with the German language may find the essential facts in Ehrlich's Croonian 

 Lecture, Proceedings of the Royal Society of London, 1900, lxvi, p. 424, and in 

 Welch's "Huxley Lecture," Medical News, igo2, lxxxi, 2, p. 721 



