THE FACTOR OF IMMUNIZATION 209 



The results indicate that the four strains can be ranked in the 

 order DBA/ 2, ST/ A, A and AKR in respect to increasing F.I., 

 whereas the order is reversed in respect to antigenic strength. 



The findings are discussed in relation to clonal selection theories 

 of immunity and tolerance. A new version of the clonal selection 

 hypothesis is proposed as the most satisfactory explanation for the 

 findings. 



Irrespective of the theoretical interpretation, the main conclu- 

 sion is that "antigenic strength" in transplantation biology is 

 primarily dependent on the initial reactivity to the antigen. 

 Given sufficient time to provoke the immune response, antigens 

 are not very different in strength. 



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