ACQUIRED IMMUNITY 75 



plished in 1890-1892 by Behring and Kitasato, 58 and by Behring 

 and Wernicke. The results of this work the direct outcome of 

 their success in actively immunizing with soluble toxins, is sum- 

 marized in their first paper as follows: "The blood of tetanus- 

 immune rabbits possesses tetanus-poison-destroying properties ; these 

 properties are demonstrable in the extravascular blood and in the 

 serum obtained from this ; these properties are of so lasting a nature 

 that they remain active in the bodies of other animals, so that one 

 is enabled to obtain positive therapeutic results by transfusing the 

 blood or injecting the serum. These tetanus-poison-destroying prop- 

 erties are absent from the blood of non-immune animals, and when 

 the tetanus poison is inoculated into normal animals it can be dem- 

 onstrated as such in the blood and other fluids of these animals after 

 death.' 7 



With these researches begins the therapeutically practicable 

 method of passive immunization which is now in such widespread 

 and successful use in the treatment of diphtheria, in the prophylactic 

 treatment of tetanus, and, to a less common and less successful de- 

 gree, in the treatment of dysentery, typhoid fever (Besredka), 

 plague, and a number of other bacterial diseases. The same method 

 has been successful in the treatment of various diseases of domestic 

 animals. The principle was also applied by Ehrlich 59 to ricin and 

 crotin immunity, in the formulation of which he succeeded in work- 

 ing out passive immunization on a quantitative basis, showing that 

 the degree of immunity in such cases could be directly referred to 

 the amounts of the specific antitoxin present in the blood of the im- 

 munized animal. Calmette, 60 and Physalix and Bertrand, 61 then 

 succeeded in producing passive immunization against snake venoms. 



To summarize the success of passive immunization in general we 

 may say that it has achieved its greatest usefulness in the case of 

 those diseases in which the pathogenesis depends upon a true exo- 

 toxin which, as we have mentioned before, leads to the formation 

 of an antitoxin in the immunized animal. In these cases the passive 

 immunization is accomplished by the transfer of the antitoxins from 

 the treated to the normal animal. 



In the case of bacterial infections in which no true toxin .is H 



formed where no antitoxin results and the immunity depends, as 

 we shall see, upon an enhancement of the bactericidal and phagocytic 

 properties of the blood and the cells, passive immunization has not 

 been a practical therapeutic success. The probable reasons for this 

 cannot be properly discussed until we have examined more closely 

 into the mechanism by which the immune animal is protected after 



58 Behring and Kitasato. Deutsche med, Woch., No. 49, 1890. 



59 Ehrlich. Deutsche med. Woch, 1891 ; Fortschr. d. Med,, p. 41, 1897. 



60 Calmette. Compt. rend, de la soc. de biol., 1894. 



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



