292 INFECTION AND IMMUNITY 



animal or human being in considerable quantities, without producing 

 anything more than the temporary local disturbance following the sub- 

 cutaneous administration of any innocuous material. Other bacteria, 

 on the other hand, such as the bacillus of anthrax or the bacillus of 

 chicken cholera, injected in the most minute dosage, may give rise to 

 a rapidly fatal septicemia. Within the same species, furthermore, 

 fluctuations in virulence may take place which may depend upon a 

 variety of influences which have been discussed in another section and 

 need not be recapitulated. Suffice it to say that variations in the sus- 

 ceptibility of inoculated subjects do not, in any way, furnish a sufficient 

 explanation for these phenomena. 



In an effort to cast light upon this subject, Bail, following in the 

 footsteps of his predecessors, Kruse, 1 Deutsch and Feistmantel, 2 has 

 formulated his so-called "aggressin-theory." 



Bail 3 was first led to the formulation of his theory by extensive re- 

 searches which he had made in conjunction with Petterson 4 into an- 

 thrax immunity. He had noted, as others before him had, that animals, 

 highly susceptible to anthrax, often possessed marked bactericidal 

 powers against this bacillus. When such animals, whose serum should 

 surely be capable of bringing about the death of, at least, a few hundred 

 anthrax bacilli, were injected with doses far less than this number they 

 nevertheless succumbed rapidly and the bacilli multiplied enormously 

 in their bodies. He argued from this that the injected microorganisms 

 must possess some weapon whereby they were enabled to counteract 

 the protective forces of the animal organism. In an anthrax-immune 

 animal, as a matter of fact, no proliferation of bacteria took place and 

 the injected germs were rapidly disposed of by the protective forces, 

 foremost of which was phagocytosis. 



The theory of Bail 5 contains the following basic principles : 6 



Pathogenic bacteria differ fundamentally from non-pathogenic bac- 

 teria in their power to overcome the protective mechanism of the ani- 

 mal body, and to proliferate within it. They accomplish this by virtue 

 of definite substances given off by them, probably in the nature of a 

 secretion, which acts primarily by protecting them against phagocy- 



1 Kruse, Ziegler's Beitrage, xii, 1893. 



2 Deutsch und Feistmantel, "Die Impfstoffe in Sera," Leipzig, 1903. 



3 Bail, Cent, f . Bakt., I, xxvii, 1900, and xxxiii, 1902. 



4 Bail und Petterson, Cent. f. Bakt., I, xxxiv, 1903; xxxv, 1904; xxxvi, 1904. 



5 Bail, Arch. f. Hyg., lii, 1905; liii, 1905; Wien. klin. Woch,, xvii, 1905. 



6 Bail und Weil, Wien. klin. Woch., ix, 1906; Cent. f. Bakt., I, xl, 1906; xlii, 1906. 



