144 THE DIFFERENT TYPES OF IMMUNITY 



receptors were to occur either in all cells or only in those cells which 

 are susceptible to toxin influence. As a matter of fact, there is 

 experimental evidence to show that this may occur. Thus, w y hile 

 the red-blood corpuscles of the normal rabbit are readily destroyed 

 by the peculiar toxin which is found in the serum of eels, the cor- 

 puscles of correspondingly immunized animals, even though washed 

 free from any antitoxin that may be present in the serum, are abso- 

 lutely resistant. Evidently they have lost the receptors which in 

 the non-immune animal made the action of the toxin possible. It 

 has similarly been observed that animals which have been highly 

 immunized against diphtheria toxin may finally cease the produc- 

 tion of antitoxin altogether, and simultaneously lose their suscep- 

 tibility to the toxin in question altogether, phenomena which are 

 most readily explained upon the basis of acquired atrophy of the 

 corresponding receptors. 



While these examples plainly illustrate the undoubted occurrence 

 of an acquired antitoxic immunity, due to receptoric atrophy, there 

 are further observations which show that this principle also plays 

 an important role in the development of other types of immunity. 

 Thus far we have only considered the reaction of the macroorganism 

 to the introduction of the microorgansim, and the question very 

 naturally suggests itself, Is it not possible that the microorganism 

 may become resistant to the deleterious influences which it meets 

 within its host? We have seen that it may protect itself by the 

 development of capsules and the liberation of aggressins. Within 

 recent years, observations have come to light, however, which make 

 it very probable that the principle of receptoric atrophy may play 

 an important role here also. Much of this work and its brilliant 

 interpretation we likewise owe to the genius of Ehrlich. He has 

 shown that on treating rats which have been infected with trypano- 

 somes (*SI) with an amount of arsenophenyl glycin, arsanil, or 

 arsacetin, not quite sufficient to kill all the organisms, trypanocidal 

 antibodies are produced, which Ehrlich conceives to be the outcome 

 of the antigenic effect of the ordinary nutriceptors 1 of the parasite 

 upon those cells of the macroorganism which are provided with cor- 

 responding haptophoric groups. Those trypanosomes which have not 

 been killed by the arsenic now find themselves in the presence of 



1 Nutriceptors are here understood to be those receptors which serve the 

 nutrition of the organism (sc., the cell). 



