86 THE BACTERICIDAL SUBSTANCES OF THE BLOOD 



Offensive-defensive Mechanism in Infections with Semiparasites. 

 If now we turn our attention to the offensive-defensive mechanism 

 which is thrown into operation in infections with the so-called semi- 

 parasites, of which the typhoid bacillus and the cholera vibrio are 

 typical examples, we meet with still a different picture, which is 

 fairly well defined also, although it has not been worked out in its 

 details so thoroughly as we have seen it in anthrax. A great deal 

 again depends upon the quantitative relations at the point of infec- 

 tion. If the infecting dose (of the cholera vibrio, for example, given 

 intraperitoneally) is large, e. g., several multiples of the quantity 

 which will just produce infection, there is virtually no evidence of a 

 defensive reaction. The organisms multiply from the start, or at 

 least do not diminish in number even during the first few hours; 

 there is no evidence of phagocytosis or of extracellular degeneration. 

 Leukocytes indeed are relatively scant, while the abdominal cavity 

 is filled with a serous exudate, in which the bacteria multiply as in 

 an ordinary culture medium. The animal at the same time shows 

 evident signs of being ill; the abdomen is tense and exceedingly 

 tender, the hair is ruffled, the temperature drops, and death soon 

 results. 



From such a picture one would be led to conclude that the animal 

 was devoid of all defensive means against the organism in question. 

 This, however, would be erroneous, for on injecting another guinea- 

 pig with a much smaller dose, e. g., one-half the minimal infecting 

 dose, which after all represents an enormous number of bacteria, 

 the findings will be altogether different. If specimens of the peri- 

 toneal contents are removed at various intervals after the injection, 

 it will be observed at a very early period that active bacteriolysis 

 is already going on which may indeed be so extensive that after one 

 hour the peritoneal cavity may have become microscopically free from 

 organisms. But even if this does not result, the destruction of 

 bacteria is in any event very considerable, and becomes complete 

 through the introduction of a new factor, viz., the appearance of 

 large numbers of leukocytes which are mainly of the polynuclear 

 neutrophilic type. These dispose of the remaining organisms by 

 phagocytosis, and the peritoneal cavity finally becomes sterile. 



This means, in other words, that the animal which showed no 

 evidence of a defensive reaction in the first experiment, actually 

 had a very definite mechanism of this kind at its disposal, and the 



