IMMUNOLOGY 869 



to subject this strain to immune serum and to secure a relapse strain 

 which showed the zone phenomenon with one to five parasites per 

 field. Such data give a basis for the interpretation of the variable and 

 often contradictory results obtained by investigators doing only a 

 few tests. 



The relapse strains can be differentiated from the passage strain not 

 only by their resistance to lysins, but by their behavior in other serologi- 

 cal tests, such as the Rieckenberg blood platelet test (Rieckenberg, 

 1917; see also Brussin and Kalajev, 1931). They also differ antigeni- 

 cally and therefore stimulate different immune mechanisms, as is 

 shown, for example, by cross-immunity tests. 



The difference in antigenic constitution of various strains was origi- 

 nally studied by Ehrlich and his coworkers in infections in mice in 

 which artificial crises were produced by incomplete cures with drugs. 

 Of the earlier papers, that of Ritz (1914) is particularly interesting. 

 He incompletely cured a mouse twenty times, during which seventeen 

 immunologically different relapse strains were produced, as tested by 

 cross-immunity in mice after cure. Some of these strains were identical 

 with those of another mouse which had been incompletely cured nine- 

 teen times, during which nine immunologically different strains had been 

 produced. The immunological variations may be inherited, but in time 

 may be lost. Ritz (191 6) also showed that the strains arising naturally 

 in the rabbit could be differentiated by the same methods. In the suc- 

 ceeding years, more or less similar studies have been made with both 

 antibody and drug-induced relapse strains. Recently, Lourie and O'Con- 

 nor (1937), in an in vitro study of relapse strains after drug treatment, 

 obtained twenty-two relapse strains of which thirteen were immuno- 

 logically distinct. In addition, they ascertained that certain strains tended 

 to occur more frequently than others, that a strain may be a combina- 

 tion of two or several strains, and that individual strains may disappear 

 from such compound strains. 



The acquisition of this antibody resistance by the trypanosomes, with 

 a concomitant antigenic change, is an interesting case of an environ- 

 mentally induced persistent modification which is inherited for many 

 asexual generations, sometimes through 400 mouse passages. It seems 

 to be similar to the acquisition of drug resistance by free-living Proto- 

 zoa. It can be produced not only in vivo, but also in vitro. It is al- 



