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. By artificial atten- 

 uation of these living virulent bacteria Pasteur succeeded in obtaining 

 vaccines of several of them. The methods that he employed were, however, 

 totally impracticable, for not infrequently, by the use of the vaccine, the 

 disease which it was the object to prevent was instigated. It was there- 

 fore 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 "aggressins"; i.e., exudates from animals that had 

 been infected with the respective bacteria. 



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

 He believes that during an infection, the bacteria secrete certain agents which counter- 

 act or entirely destroy the infected organism's protective powers, especially phago- 

 cytosis. 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 aggres- 

 sins. If this theory be correct, it should be possible to demonstrate aggressins, espe- 

 cially 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 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 giunea-pigs, by subcutaneous inoculation, 

 a half parasite. 



The Obtaining of Aggressins. 



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

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

 manner. 



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