86 INFECTION AND RESISTANCE 



by injecting antitoxic sera is almost immediately established ; that, by 

 proportionately increasing the amount of antitoxin, immunity can 

 be produced against any amount of toxin; and that this passive or 

 transferred immunity is of relatively short duration. 



The antitoxins, then, as we shall see in the more detailed analysis 

 of their action (in chapter V), are specific poison-neutralizing anti- 

 bodies formed in the blood of animals immunized with a true bac- 

 terial toxin or exotoxin conferring resistance or immunity, not by 

 influencing the bacteria, but by rendering innocuous the specific bac- 

 terial poisons. 



The therapeutic successes of passive immunization achieved with 

 tetanus and diphtheria very naturally led to a careful inquiry into 

 the antitoxic properties of the blood of animals immunized with all 

 known pathogenic bacteria and bacterial products, and with many 

 poisons of animal and vegetable origin. 



Contrary to earlier expectations, however, the list of bacteria 

 against which antitoxic immunity can be achieved has remained rela- 

 tively small, limited in fact, as we have previously stated, to those 

 species which produce a soluble exotoxin. The inciting of a specific 

 neutralizing antibody (antitoxin), however, is also a property of 

 many other substances of proteid nature which are for this reason 

 classified biologically with the true toxins or exotoxins. In fact, 

 the one absolutely constant attribute which defines our conception 

 of the "true toxins" and the substances classified with them is their 

 antitoxin-inciting power. We classify a bacterial product as a 

 "toxin" or "exotoxin" only if it incites a neutralizing "antitoxin" 

 in the serum of an immunized animal. 



The first discovery of a non-bacterial antitoxin-stimulating sub- 

 stance was, as we have stated, that of ricin by Ehrlich, 24 1891, and 

 this was soon followed by similar determinations for abrin and robin 

 other vegetable poisons. In 1894 Calmette, 25 and Physalix and 

 IBertrand 26i extended the principle to poisons of animal origin by 

 demonstrating antitoxin formation against snake poison. And that 

 similar specific neutralizing bodies were formed in response to im- 

 munization with ferments was shown in 1900 by Morgenroth. 27 



The more important individual substances which may be bio- 

 logically grouped together because of their property of inciting a 

 specific antitoxin (or toxin-neutralizing body) in the blood of im- 

 munized animals may be tabulated as follows : 



Diphtheria toxin (loc. cit. Behring & Wernicke). 

 Tetanus toxin (loc. cit. Behring & Kitasato). 



24 Ehrlich. Deutsche med. Woch., 1891; Fortschr. d. Med., 1891, 1897. 



25 Calmette. Ann. Past., Vol. 8, 1894. 



28 Physalix and Bertrand. Compt. rend, de la soc. de biol., 1894. 

 27 Morgenroth. Centralbl. f. Bakt., 26, 1899. 



