138 THE DIFFERENT TYPES OF IMMUNITY 



corresponding haptophoric groups. Those trypanosomes which have 

 not been killed by the arsenic now find themselves in the presence 

 of these antibodies (A I), and in so far as they are not destroyed, they 

 respond to the occupation of their original nutriceptors (N I) by these 

 antibodies with the production of a new type of nutriceptors (Nil), 

 which we may imagine to possess a greater affinity for the available 

 foodstuffs than for the antibodies that are simultaneously present. 

 A new strain of trypanosomes thus develops in which this peculiarity 

 is handed down from each individual parasite to its descendants. If 

 this strain (S II) is now tested against a serum containing antibodies 

 of the type A I, it will be found immune, and as Ehrlich has pointed 

 out this type of immunity can be explained only in the manner 

 just outlined, viz., on the basis of receptoric atrophy. 



The importance of this principle in the interpretation of various 

 phases of human and animal pathology is, of course, evident. It 

 readily explains, for example, why the syphilitic individual is refrac- 

 tory to reinoculation, while he is liable to relapses starting from his 

 original infection. We may imagine that in such a person, different 

 strains of spirochetes develop as a result of adaptation to those 

 antibodies which are formed in consequence of the death and absorp- 

 tion of the first and subsequently developing strains, and that the 

 latest strain, in point of time of development, will always be capable 

 of causing a relapse, as no suitable antibodies to it have as yet 

 developed and because it is immune to those that have been formed 

 before. The number of strains which can theoretically be produced 

 in the course of an infection will, no doubt, vary with different 

 organisms, as well as with the nature of the host. A great deal of 

 additional work will have to be done, however, before we can speak 

 with any degree of definiteness on this subject. Ehrlich has shown 

 that in the case of the spirillum of relapsing fever only three or four 

 strains are possible. If, then, the patient or animal has had two or 

 three relapses the body will contain all the different "strains" of 

 spirillocidal antibodies that are possible, no new strain can accord- 

 ingly develop, and spontaneous recovery will occur. The greater 

 the number of strains which can develop the greater will naturally 

 be the obstacles to spontaneous recovery. This holds good espe- 

 cially for such diseases as syphilis and trypanosomiasis (sleeping 

 sickness), and possibly also for malaria. 



From these brief considerations it will be seen that the subject 



