206 MORTEN SIMONSEN 



the "strong" test systems employed in the present work, may be 

 rendered highly tolerant witliin 24 hours after grafting. If this is 

 so, tolerance is probably also induced in the weaker test systems, 

 and presumably even faster. 



The possibility can therefore not be dismissed that part of the 

 grafted cell population is rendered tolerant before it has succeeded 

 in producing the damage necessary for production of spleno- 

 megaly. If, however, the assumption is correct that tolerance in 

 grafted cell populations is induced faster in response to weak than 

 to strong antigens, this fact might furnish an alternative explana- 

 tion of the different Factors of Immunization. 



It could now be postulated that : 



{a) There need not be a clonal selection on a cellular basis ; all 

 cells may maintain potential reactivity to all antigens provided 

 they are adequately stimulated, as they are presumed to be in a 

 graft-versus-host reaction. 



(fc) Strong antigens do not allow production of tolerance against 

 them in the time required by the grafted cells to be stimulated and 

 hence to initiate a graft-versus-host reaction. Virtually nothing is 

 gained, therefore, by preimmunizing the donor; hence a low F.I. 



{cj Weak antigens, on the contrary, induce a considerable 

 degree of tolerance in the graft. A larger dose of normal cells has 

 to be injected in order to obtain a similar degree of spleen enlarge- 

 ment (cf. Table II, column 6). 



(J) Preimmunization of the donor renders the cells relatively 

 refractory to the induction of tolerance (of which I have some 

 independent, but incomplete evidence). While this is immaterial 

 in a strong antigenic combination, it allows the graft in a weak 

 combination to preserve its reactivity until the spleen has been 

 damaged; hence a high Factor of Immunization. 



With either explanation, it is assumed that (i) the graft-versus- 

 host reaction which results in splenomegaly takes place in a 

 relatively short time (probably mostly within the first day, c£, 

 Simonsen, 1962), and (2) that the cells which cause splenic 



