52 EXPERIMENTAL RESEARCHES 



parasites and mixed their bloods with solutions 

 of trypocid of different degrees of concentration. 

 I now observed the very unexpected phenome- 

 non that in these mixtures the immune strain was 

 far less resistant to higher doses of trypocid than 

 the normal one. Thus whilst concentrations of 

 i in 500 to i in 1,000 almost instantaneously 

 killed the immune strain, the control strain 

 retained its motility unimpaired in these con- 

 centrations for not less than five minutes. 



We have here, therefore, one of those striking 

 phenomena which have already been met with 

 in the study of immunity and which consist in 

 the simultaneous occurrence of immunity and 

 hypersensibility in one and the same organism ; 

 but the case here is somewhat more simple, 

 since we are working with unicellular organisms, 

 and may therefore hope to obtain a more accurate 

 idea of the case. 



Allow me at this juncture to suggest to you a 

 few fundamental principles. In order that a 

 given poison, e.g., the arsenical preparation, 

 may act upon the trypanosomes, it must contain 

 certain chemical groups which seize upon the 

 latter. These groups I will distinguish from 

 the ordinary receptors, which play an important 

 part in the theory of immunity, by the name of 



