4 THE PRINCIPLES OF IMMUNOLOGY 



organisms resistant to the defensive phenomena, phagocytosis and 

 agglutination, as will be discussed in subsequent chapters. There- 

 fore, capsule formation may well constitute an aid to invasion. 



Aggressins. It was found by Koch that tuberculous animals 

 injected intraperitoneally with fresh cultures of tubercle bacilli suc- 

 cumb soon after the injection, and that a considerable amount of 

 exudate appears in the peritoneum. This phenomenon seems to 

 have been the basis of Bail's aggressin theory. Bail injected tubercle 

 bacilli, together with sterile tuberculous exudate, into healthy 

 guinea-pigs and found that the injected animals died in the course 

 of twenty-four hours, while control animals inoculated with the 

 exudate alone did not show any appreciable effect, and control 

 animals which received tubercle bacilli alone died only after the 

 lapse of several weeks. He argued from this that the sterile exudate 

 must contain a substance or substances which are responsible for the 

 increased virulence or aggressiveness of the bacilli. He named this 

 substance " aggressin " and believed that during an infection the 

 organisms secrete certain substances which have power to inhibit 

 or destroy the protective powers of the host. These bodies are sup- 

 posed to be formed by the living bacteria in the living body only, 

 and the pathogenicity of bacteria is said to depend, in part at least, 

 upon their ability to produce aggressins. Bail believed further that 

 the germicidal activity of body fluids in natural immunity had been 

 overemphasized. He had noted with Petterson that animals highly 

 susceptible to anthrax often possessed sera which had marked bac- 

 tericidal powers against the anthrax bacillus. If such animals were 

 inoculated with a few hundred organisms, a number easily destroyed 

 by their sera, they nevertheless rapidly succumbed to the disease. 

 Bail also showed that the peritoneal fluid of guinea-pigs dying after 

 a fatal injection of typhoid or cholera organisms possessed the 

 ability to increase the virulence or infectivity of particular strains 

 that would otherwise have been harmless. Experiments of this 

 kind were also performed in dysentery, chicken cholera, pneumonia, 

 and staphylococcus infections, and the results obtained were identi- 

 cal with those observed in the case of tubercle bacillus. Heating 

 the exudate to 60 C. instead of inhibiting, increased the aggres- 

 siveness of the organisms. Small doses appeared to act relatively 

 more strongly than larger doses. In tuberculous animals the tissues 

 seemed to be saturated with this body, and when fluid collected in 

 the body cavities, as happens on injection of tubercle bacilli, these 

 fluids contained large quantities of aggressins capable of inhibiting 

 phagocytosis by preventing the migration of polymorphonuclear 

 leucocytes. Bail, however, was not the first to observe this par- 

 ticular phase of bacterial offense. Salmon and Smith, as early as 

 1884, noted that bacteria multiply in the tissues of their host be- 

 cause of a poisonous principle which is produced during their 

 growth and multiplication. Kruse maintained that the organisms 

 secrete ferment-like bodies (referred to as " lysins ") which have 



