124 INFECTION AND RESISTANCE 



gether. This was confirmed in 1904 by von Dungern 26 for diph- 

 theria toxin, and Craw 27 was able to observe it in the case of mega- 

 theriolysin and its antilysins. 



Von Dungern interpreted this in the sense of Ehrlich, by assum- 

 ing it to be due to what he calls "epitoxonoids." This epitox- 

 onoid he assumes to be a constituent of toxic broth, which has still 

 less affinity for antitoxin than the toxon. It can combine with diph- 

 theria antitoxin, but not until all the true toxin is bound. However, 

 when it is once united with diphtheria antitoxin it is not very easily 

 displaced from the union, especially when a considerable time has 

 elapsed since the union. Therefore, he thinks, when the toxin is 

 added to the antitoxin in two fractions, this epitoxonoid is bound and 

 keeps the toxin, which is added later, out of combination. Whereas 

 if the toxic broth is added as a whole, it is the epitoxonoid which is 

 left unbound. This explanation of von Dungern's may be looked 

 upon as an ingenious refinement of the reasoning introduced by Ehr- 

 lich into this field. As a matter of fact reactions similar to the 

 Danyz phenomena have been very commonly observed in the reac- 

 tions between various colloids. 



THE SIDE-CHAIN THEORY 

 Mechanism of Antibody Formation 



The discovery of antitoxins in the blood serum of toxin-immune 

 animals by Behring and his collaborators furnished a point of new 

 departure for the investigation of the phenomena of immunity, and 

 Ehrlich's work upon the nature of the reaction between toxin and 

 antitoxin, both in vitro and in the animal body, firmly established 

 that the protective effect of the latter was one of direct neutraliza- 

 tion, and not, as at first supposed, one of toxin destruction or of 

 indirect influence through the mediation of the body cell. As we 

 have seen, moreover, it was quickly noted that these reactions were 

 strictly specific in that an antitoxin produced with any one of the 

 known toxins reacted solely with this one to the exclusion of all 

 others. All these facts were of the utmost practical importance and 

 gave hope of ultimate extensive therapeutic application, a hope 

 which has, in part, been realized. 



The physiological mechanism by which these phenomena were 

 brought about, however, was, and is, to a great extent still, a mys- 

 tery, and a most extensive and painstaking series of researches has 

 occupied itself with its elucidation. 



26 Von Dungern. Deutsche med. Woch., 30, 1904. 



27 Craw. Jour. Hyg., Vol. 7, 1907. 



