INFECTION AND IMMUNITY i6f 



site are concerned entirely with the parasite's nutri- 

 tion. Owing to the destruction brought about by 

 the chemical agent, some of these receptors pass 

 into the monkey's body, and, acting as antigens, 

 excite the production of antibodies directed against 

 these particular receptors. When living parasites 

 are brought into contact with this antibody, either 

 in vitro or in vivo, the antibody is anchored by 

 the parasites. As a result of this occupation of 

 its receptors, the parasite undergoes a biological 

 alteration which consists in the disappearance of 

 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 becomas 

 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 



