i8o IMMUNE SERA 



In his researches on trypanosomes Ehrlich found 

 that if the dose of germicide administered to an 

 infected animal was too small to kill all the para- 

 sites, there developed after a time a strain of organ- 

 isms which were resistant to the further action of 

 the germicide. It was usually futile to repeat the 

 dose, for the resistant parasites would survive. 

 At the same time the interesting observation was 

 made that this resistance manifested itself only 

 in the animal body; in vitro the parasites could 

 still be filled by the germicide in question. It was 

 also found that the various chemical substances 

 which exerted therapeutic effects in animals infected 

 with trypanosomes could be grouped into three 

 classes, namely i, various arsenicals (arsenious 

 acid, atoxyl, arsacetin, arsenophenylglycin, and 

 finally "606,"); 2, certain azo dyes (among them 

 trypan red, trypan blue, and trypan violet) ; and 3, 

 certain basic triphenylmethane dyes (among them 

 parafuchsine, methyl violet, pyronine, etc.). Against 

 each of these classes it was possible to produce speci- 

 fically resistant strains of trypanosomes, so that a 

 strain which had been made resistant to fuchsin was 

 also resistant to related basic dyes, but vulnerable 

 to the azo dye and to the arsenicals. Moreover, 

 by appropriate treatment it was found possible 

 to produce strains resistant against all three classes 

 of trypanocidal agents. 



According to Ehrlich the union of the chemical 



