152 



HOWARD, MICHIE AND WOODRUFF 



between tolerance and immunity may be decided, for a given an- 

 tigenic difference, by one variable only, regardless of the animal's 

 age, namely, the ratio of antigen dosage to the size of the target. 

 In Table VI we have set out some rough quantitative com- 

 parisons between the newborn and the adult, from which we 

 conclude that the latter's spleen contains about 200 times as many 

 lymphocytes as that of the newborn. If we can take the spleen as 

 representative of lymphoid tissue, and the lymphocyte as repre- 

 sentative of the immunologically competent cells involved in 



.*. Calculated factor of increase in total no. of lymphocytes in spleen=6-5X i-ox 



I-yX 20-0=220. 



homograft reactions, we fmd a remarkably good agreement 

 between the predictions of our hypothesis and the observed facts. 

 Although the closeness of fit may be fortuitous in detail, we fmd 

 encouragement in the fact that two independent assessments lead 

 to ratios of the same order of magnitude. 



This simple scheme implies that the induction of tolerance 

 could be facihtated by the use of agents which reduce the total 

 size of the host's lymphoid cell population. An interpretation 

 along these lines might be applicable to the effect of prior irradia- 

 tion revealed by the study on adult mice reported in this paper, 

 and also to Woodruff's (1957) observation that treatment with 



