THE FACTOR OF IMMUNIZATION 



203 



different loci, which fact might reflect that immunological 

 reactivity is more essential for the organism in respect to some 

 antigens than to others. Some loci would never lose their active 

 genes. If so, the antigens against which the activity of these genes 

 is directed would be obhgatory in the sense that all immuno- 

 logically competent cells would react against them, given optimal 



DBA/2 



Fig. 2. Clonal selection on basis of loss mutations. 



conditions for stimulation by the antigen. Other antigens would 

 be more or less facultative in the sense that the genes which 

 determine reactivity against them have been lost in a smaller or 

 greater part of the entire cell population. 



Facultative antigens, upon their injection, would therefore lead 

 to clonal selection in the sense which Burnet originally visuahzed, 

 while obhgatory antigens would not. Fig. 2 is intended to repre- 

 sent a crude illustration of this hypothesis as applied to the four 

 antigens of the present investigation. The cells I-IV symbohze 



