ACTIVE AGGRESSIVITY 39 



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 question arises : What 

 evidence have we that such forces may actually be 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 multiple 

 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 sub- 

 fatal doses of the same organism into fatal doses; in other words, it 

 had acquired properties which evidently favored infection. Bail sup- 

 posed 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 intq 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, intra- 

 peritoneally 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 

 hours the exudate is thick, tenacious, milky looking, and is composed 

 almost entirely of polynuclear leukocytes which have taken up many 

 or all of the injected organisms according to the number which were 

 originally introduced. In A, on the other hand, the fluid is abundant, 



