IMMUNITY AND SUSCEPTIBILITY. 139 



probability that the endothelial or other cells had any- 

 thing to do with it. He, however, concludes that the 

 phenomenon depends upon the presence of substances 

 derived from leukocytes, the only cells found in perito- 

 neal fluid. Bordet modified Metschnikoff 's methods and 

 used a suspension of cholera germs in bouillon. Two 

 drops of this were added to one drop of anticholera serum 

 and mixed. One drop of the mixture and one drop of 

 normal guinea pig's serum were then brought in contact 

 in a hanging-drop and examined microscopically. He 

 found that in all cases in which the PfeifFer phenomenon 

 would have taken place within the animal's body it 

 took place in his artificial preparation. 



The explanation of Pfeiffer's phenomenon is not at all 

 easy, but Kruse x has suggested a theory based upon the 

 assumption that substances described as lysins are formed 

 by the bacterial growth. These substances are of the 

 nature of ferments, and attack and destroy the alexins, 

 thus enabling the pathogenic bacteria to overcome the, 

 bacteria-destroying mechanism of the tissues. Kruse sees 

 in the process of immunization the formation of antilysin, 

 by which the powers of the lysins are destroyed, and the 

 alexins once more enabled to exert an active destructive 

 influence upon the bacteria. 



That the germicidal substance is derived from the 

 leukocytes is not improbable. Laschtschenko 2 found 

 the action of heterogeneous serum very marked in liber- 

 ating the alexins. This subject has, however, already 

 been discussed {vide supra). 



Explanation of Acquired Immunity. — 1. The Ex- 

 haustion Theory of Pasteur and Klebs. — This 

 theory is of historic interest only. It was suggested in 

 1880 by Pasteur 3 and Klebs,* who thought that when a 

 bacterium developed in the body it used up some sub- 



1 See Flugge's Die Mikroorganismen, 1892, vol. i., p. 414. 

 3 Munch, med. Wock., 1899, No. 15. 



3 Compte-rendus de la Soc. Biol, de Paris, xci. 



4 Archivfur Experimented Path. u. Phannakol.. xiii. 



