126 IMMUNITY AND INFECTION 



base unite; the chemical analogy of toxin-antitoxin union to form 

 an inert mixture comparable to a salt was further accentuated by the 

 effect of moderate degrees of heat in hastening the reaction between 

 the two. A very thorough investigation of the quantitative neutraliza- 

 tion of toxin by antitoxin revealed the error of this supposition and 

 Ehrlich was led to assume a very complex structure for the toxin mole- 

 cule, in which there existed several fractions possessing individually, 

 different affinity for antitoxin. 



Madsen and Arrhenius 1 studied the toxin-antitoxin union from 

 the standpoint of physical chemistry and found that the slightly dis- 

 sociated reactive substances united in conformity with the law of 

 mass action of Guldberg and Waage. Their conclusion was that 

 toxin and antitoxin react like a weak acid and weak base, and that it 

 is a reversible reaction, so that a mixture of toxin and antitoxin always 

 contains free toxin," free antitoxin and toxin-antitoxin, the relative 

 amounts being calculable according to the law of mass action. The 

 observations of Theobald Smith 2 and of many other observers that 

 neutral mixtures of toxin and antitoxin would induce active immunity 

 in experimental animals are in harmony with this view. Biltz 3 has 

 advanced an hypothesis, based upon the assumption that toxin and 

 antitoxin are colloids, which in essence assumes that the toxin-anti- 

 toxin reaction is a phenomenon of adsorption, quite unlike the reaction 

 of a weak acid and a weak base. 



The humoral theory of immunity fails to attribute to phagocytic 

 cells any prominent part in immunity. No theory has been advanced, 

 up to the present time, which explains all the phenomena of humoral 

 immunity; whatever the final solution may be, the side-chain theory 

 as developed and defended by Ehrlich must, and always will be, a 

 worthy monument to a great man. 



B. The Cellular or Phagocytic Theory of Immunity. The cellular 

 theory of immunity, formulated and championed by Metchnikoff, 

 had its inception in observations of the nutritive activities of amebse, 

 which could be watched under the microscope. It was observed 

 that these simple, transparent protozoa, when about to feed, ap- 

 proached and flowed around a minute organism, as a bacterial cell. 

 Shortly after engulfment the contour of the ingested bacterium lying 

 within the substance of the ameba became less and less distinct and 



1 See Arrhenius, Immunochemie, Leipzig, 1907, for full details. 



2 Jour. Exp. Med., 1909, xl, 241, Active Immunity Produced by So-called Balanced 

 or Neutral Mixture of Diphtheria Toxin and Antitoxin. 



3 Ztschr. f. physiol. Chem., 1904, 615. 



