1 66 IMMUNE SERA 



successfully overcomes an infectious disease without 

 entirely overcoming the infecting bacteria. This is 

 well shown by what we call chronic germ carriers. 

 Deutsch regards the increase in virulence brought 

 about by successive passage of a bacterium through 

 a susceptible animal as representing an immunity 

 developed by the bacterium against the anti- 

 bacterial agencies of the body. 



Atrepsy. Ehrlich has investigated this phe- 

 nomenon in the case of trypanosomes. He found 

 that a monkey which had been infected with a 

 particular strain of trypanosome and then cured 

 by means of chemo- therapeutic agents, when 

 tested with the original strain was not immune, 

 the disease reappearing after a long incubation. 

 If mice were inoculated with blood from the diseased 

 animal, i.e., with blood containing trypanosomes, 

 they became infected and died. Curiously, how- 

 ever, if the trypanosomes were first removed from 

 this monkey blood, it was found that the serum 

 was able to kill the original strain of trypanosomes. 

 This showed that the trypanosomes had undergone 

 some change in the body of the monkey; they 

 differed from the original strain in their behavior 

 toward the serum; they had become " serum-fast." 

 Similar observations were made at the same time 

 by Kleine, and recently also by Mesnil. 



In explanation of this adaptation, Ehrlich sug- 

 gests that certain particular receptors of the para- 



