184 IMMUNE SERA 



the original receptor group and its replacement 

 by a new group. Ehrlich's researches lead him to 

 believe that the antibody has merely an anti- 

 nutritive action, blocking the nutrireceptor of the 

 parasite and so bringing about starvation. The 

 parasite thus develops immunity by getting rid 

 of certain of its nutrireceptors, and replacing them 

 with different ones. This form of immunity Ehrlich 

 speaks of as " atrepsy," while the antibodies de- 

 veloped against the nutrireceptors he terms " atrep- 

 sins." A somewhat different example of atrepsy 

 is the following: Bird-pox, virulent for both fowl 

 and pigeon, if passed through the pigeon becomes 

 completely avirulent for the fowl. Ehrlich believes 

 that the parasite in passing through the pigeon has 

 to assimilate substances different from those assim- 

 ilated in its passage through the fowl. There- 

 fore that part of the receptors which deals with the 

 nutritive substances in the fowl's organism is not 

 in use during the passage through the pigeon and 

 may become atrophied, so that on the parasite 

 being transferred back to the fowl, supposing one 

 of the specific constituents of fowls to be neces- 

 sary for its proliferation, it would no more be 

 able to grow. We have, therefore, a loss of cer- 

 tain receptors which are absolutely necessary for 

 nutrition. 



Ehrlich suggests that probably the majority of 

 so-called non-pathogenic micro-organisms, if intro- 



