54 EXPERIMENTAL RESEARCHES 



and far more economical expedient than the 

 loss of a perhaps necessary chemical structure. 

 It is quite evident that the same result could be 

 obtained if the chemo-receptors of the trypano- 

 somes were so far reduced in their affinity that 

 the proportion of distribution was altered in 

 favour of the chemo-receptors of the organism. 

 That, indeed, is what takes place. Had the 

 chemo-receptors quite disappeared, the immune 

 strains would also in the test-tube experiment 

 be more resistant than the normal ones ; as a 

 matter of fact, with certain preparations the 

 contrary occurs.* The phenomenon is most 

 easily explained by the supposition that the 

 affinity of the chemo-receptors has become 

 adjusted in such manner to the counterbalancing 

 affinity of the mouse organism, that in the mouse 

 no more arsenic remains at the disposal of the 



* The fact that these resistant strains are, in the test- 

 tube experiment, affected even more severely than the 

 normal ones, is the result of the co-existence of resistance 

 and hyper-sensitivity in the parasite's protoplasm. Such 

 a combination of immunity and hyper-sensitivity has 

 repeatedly been observed in the course of the immunisa- 

 tion of higher animals, and this is now assuming increasing 

 importance. It is the more interesting to note that the 

 same phenomenon occurs also in uni-cellular organisms. 

 The explanation of this observation will be given later. 



