208 MORTEN SIMONSEN 



It should be noted that the two hypotheses I have offered as 

 explanations of my present findings are not mutually exclusive. 

 The *' loss-mutation" hypothesis provides, at the same time, a 

 partial explanation of the "tolerance" hypothesis, in so far 

 as it accounts for the greater ease with which tolerance is induced 

 in the various strain combinations. Consequently, it also accounts 

 for the differences in Factors of Immunization. Alternatively, if 

 the uneven inducibility of tolerance has a different and, to my 

 knowledge, quite unsubstantiated mechanism, there may be no 

 need to assume a "loss-mutation" hypothesis in order to explain 

 the different Factors of Immunization. 



Whatever may represent "the truth" in the realm of immuno- 

 logical theory, it seems very clear that "antigenic strength" in 

 transplantation biology is primarily dependent on the intensity of 

 the initial reactivity to the antigen. Given sufficient time to pro- 

 voke the immune response, antigens are not very different in 

 strength. 



Summary 



The term Factor of Immunization (F.I.), is introduced to 

 signify the potency ratio between 2 suspensions of immuno- 

 logically competent cells, one derived from a normal animal, and 

 the other from a preimmunized animal. 



The reactivity of the two cell suspensions is measured by the 

 spleen assay of graft-versus-host reaction. 



The F.I. has been measured in four strain combinations with 

 C3H as the donor strain. The four kinds of recipients used in the 

 spleen assays were Fi hybrids between C3H and DBA/ 2 (H-2^), 

 ST/A (H-2^), A (H-2") and AKR (H-2^). 



The antigenic strength which these foreign strains represent 

 vis a vis C3H has been measured in two ways: by the number of 

 normal C3H cells needed to give a doubling of the spleen weight 

 in the corresponding Fj hybrids, and by the degree of runting 

 which results from injection of 25 miUion normal C3H cells. 



