MECHANISM IN INFECTIONS WITH SEMIPARASITES 85 



corresponding immune serum and of thus preventing phagocytosis 

 and bacterial destruction through this agency. 



In this respect there is a striking difference between the truly 

 infectious and the non-infectious or merely locally infectious strepto- 

 coccus, for on adding aggressin from an infectious to a non-infectious 

 strain the latter is not protected against phagocytosis. 



Offensive-defensive Mechanism in Infections with Semiparasites. 

 If now we turn out 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 of 

 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 



