292 INFECTION AND IMMUNITY 



of varying pathogenicity between different bacterial species and between 

 different races of the same microorganism. We know that certain bac- 

 teria may be injected into an animal or human being in considerable 

 quantities, without producing anything more than the temporary local 

 disturbance following the subcutaneous administration of any innocuous 

 material. Other bacteria, on the other hand, such as the bacillus of an- 

 thrax 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 susceptibility of inoculated subjects do not, in any 

 way, furnish a sufficient explanation for these phenomena and we are 

 forced to seek for the key to the problem in the activities of the bacteria 

 themselves. 



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

 footsteps of his predecessors, Kruse,^ Deutsch and Feistmantel,'' has 

 formulated his so-called "aggressin-theory." In its reasoning, this 

 theory is indirectly an offspring of Metchnikoff's phagocytic theory 

 and is, in many of its phases, antagonistic to the purely humoral con- 

 ception of immunity. 



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

 searches which he had made in conjunction with Petterson * 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. 



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. 



• Bail und Peiterson, Cent. f. Bakt., I, xxxiv, 1903; xxxv, 1904; xxxvi, 1904. 



