THE FACTOR OF IMMUNIZATION 187 



preimniunization of the graft population would therefore lead 

 only to a higher cell number, but not to a higher percentage of 

 cells with a specific immunological reactivity. 



The results indicate that some heightening of reactivity does 

 occur in response to preimmunization with transplantation 

 antigens of four other genotypes. But there are marked differences 

 in the extent to which this occurs. Furthermore, there is an 

 inverse relationship between the strength of the antigenic stimulus 

 (as measured by runting power) and the factor by which pre- 

 immunization increases the immunological reactivity of the 

 immunized spleen. The results can formally be covered by a 

 clonal selection hypothesis which differs radically from the one 

 originally postulated by Burnet. 



Material and methods 



Mouse strains 



The five strains employed in this investigation are, with their 

 H-2 groups given in parentheses: C3H (k), DBA/2 (d), ST/ A (b), 

 A (a), and AKR (k). 



General experimental design 



C3H is used as the donor strain for the graft-versus-host reaction 

 in infantile F^ hybrids between C3H and the four other strains. 

 As the degree of spleen enlargement which results in this test 

 system is, within a certain dose range, proportional to the 

 logarithm of the number of grafted cells (Simonsen, 1962), it is 

 possible to assay the relative potency of normal and preimmunized 

 donors by injecting their spleen cells in graded dosages. 



Technique of the spleen assay 



Cell suspensions are prepared in Tyrode solution from the 

 spleens of the two donors, one normal, and one preimmunized. 



