CHAPTER IV. 

 ACTIVE IMMUNIZATION. 



Immunization with Bacterial Extracts. Aggressin Experiments. 



The marked infectious nature of the organisms belonging to the class of 

 "pure parasites," makes it very difficult to produce an immunity against 

 them. They possess no sublethal dose in their living state, and if used 

 when dead, will produce no prophylactic immunity. Pasteur therefore, 

 by artificial attenuation of these living virulent bacteria, had succeeded, in 

 part, to obtain vaccines of several of them. The methods, however, that 

 he employed were totally impracticable, for not infrequently in the use of the 

 vaccine, the disease which it was the object to prevent, was instigated. It 

 was therefore a distinct and important triumph when Bail and Weil showed 

 that immunity against these parasites could be attained by using as vaccine 

 antigen, the so-called "aggressms;" i.e., exudates from animals that had 

 been infected with the respective bacteria. 



Bail's explanation of the aggressin-immimization method is entirely theoretical. He 

 believes that during an infection, the bacteria secrete certain agents which counteract 

 or entirely destroy the infected organism's protective powers, especially phagocytosis. 

 These bodies he called aggressins and they were distinguished by the fact that they 

 were formed by living bacteria, and only in the living body. According to Bail, the 

 pathogenicity of bacteria depends upon their power to produce these aggressins. If 

 this theory be correct, it should be possible to demonstrate aggressins, especially in 

 infections where the protective power of the organism is almost nil, as for example an 

 infection produced by the bacteria belonging to the group of hemorrhagic septicemia. 

 Unfortunately, in actual practice this is not so. 



The following experiment, however, gives an idea of the true nature of 

 these aggressins and how they are obtained. 



At first, an infecting agent the bacillus of swine pest, may be chosen. 

 This micro-organism belongs to the same class as chicken cholera and fowl 

 plague, and is distantly related to the human pest. For rabbits, this bacillus 

 is a pure parasite, for guinea-pigs, by subcutaneous inoculation, a half 

 parasite. 



The Obtention of Aggressins. 



One drop of a twenty-four-hour broth culture of this swine pest bacillus, 

 in 5 c.c. bouillon, is injected intrapleurally into a rabbit in the following 

 manner. 



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