42 OFFENSIVE FORCES OF INVADING MICROORGANISM 



forces on the part of the organism, among which the morphological 

 evidence of aggressivity is especially striking. 



Active Aggressivity. I have pointed out previously that in addi- 

 tion to such passively aggressive forces it is quite conceivable that 

 microorganisms may also possess certain active forces, and a great 

 deal of work has actually been done in the attempt to establish their 

 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 are actually 

 operative? 



With this problem the name 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 assump- 

 tion 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- 



