42 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



existence. The true toxins would of course suggest themselves at 

 once as such forces, but as we have seen already, the very organisms 

 in which toxin production is most striking are the least infectious, 

 and they can therefore hardly enter into consideration. We have 

 thus shown that the tetanus bacillus, for example, notwithstanding 

 its active toxin production, is practically unable to maintain its 

 existence in the body following primary infection. If, then, the 

 true toxins are eliminated as active aggressive forces, viz., as forces 

 which inhibit the action of the offensive forces of the host, the ques- 

 tion arises, What evidence have we that such forces may actually 

 be operative? 



With this problem the work of Bail will always remain intimately 

 associated. This investigator found that the peritoneal exudate of 

 animals which had been killed by intraperitoneal injection of mul- 

 tiple fatal doses of such organisms as the typhoid and the cholera 

 bacillus, upon subsequent removal of the organisms and sterilization 

 of the fluid with chemical antiseptics, was capable of transforming 

 subfatal doses of the same organism into fatal doses; in other 

 words, it had acquired properties which evidently favored infection. 

 Bail supposed that definite substances which were secreted by the 

 bacteria in the body of the infected animal, and which he termed 

 aggressins, were concerned in the production of this effect. He 

 assumed that the aggressins were substances sui generis, largely 

 upon the basis that aggressive exudates in themselves were found 

 to be non-toxic, and when injected into animals by themselves 

 were capable of preventing subsequent infection. This he explained 

 by the assumption that specific reaction products (antibodies) 

 antiaggressins are formed in consequence of the injection of the 

 aggressins, which render the latter inactive and thus prevent the 

 active invasion of the body by the microorganisms in question 

 (antiaggressin immunity). 



The aggressive character of the exudates is largely directed against 

 the phagocytes, which, like Metschnikoff, Bail regards as the only 

 true defensive elements of the invaded organism. This he demon- 

 strated by injecting two aggressin-immune animals, A and B, 

 intraperitoneally with equal doses of a suitable number of organisms, 

 A receiving, in addition, a certain amount of aggressin. After the 

 lapse of one or two hours the peritoneal fluid of B can then be shown 

 to contain large numbers of leukocytes, and at the expiration of four 



