THE FACTOR OF IMMUNIZATION 205 



had received a single immunizing injection showed a shghtly 

 increased total cell count but, if anything, rather less after immuni- 

 zation with AKR than after the two strongest antigens. 



The present experiments in relation to acquired tolerance 



Although these experiments seem to deal exclusively with 

 immunity, there is nevertheless one finding which I have been 

 unable to explain except in terms of tolerance. This is the fact 

 (Table II, colunm 8) that the "semi-platonic" slopes in the 4 

 strain combinations apparently fall in two categories, one slope 

 for the 2 stronger antigens, and another lower one for the 2 

 weaker antigens. 



This difference seems to be more than a sampling error. An 

 analysis of variance performed on the 4 slope populations has 

 shown that the 2 steeper ones are insignificantly different, and so 

 are the 2 smaller ones, while any of each group is significantly 

 different from any in the other group. 



I would not maintain that this fact is an obvious sign of toler- 

 ance. But, before trying to evolve how it might be suggestive, 

 I will stress the view which I think all workers in transplantation 

 biology share, that tolerance is easier to induce with weak than 

 with strong antigens. This fact is well illustrated in parabiosis 

 experiments with inbred mouse strains where a condition similar 

 to tolerance (at least superficially) can be achieved (see Martinez, 

 Shapiro and Good, i960, 1961). 



Returning to the slope differences, it could be argued that if 

 tolerance is induced in a substantial part of the grafted C3H cells 

 before they have settled in the spleen, it will lead to reduction of 

 their spleen enlarging capacity; furthermore, that this reduction 

 might be percentually higher for a large dose than for a small 

 dose, in which case it would lead to reduction of the slope, in 

 addition to parallel displacement of the curve. 



Simonsen (1962) has previously presented data to suggest that 

 C3H cells injected i.p. into (C3HX ST/ A) Fi, which forms one of 



