190 MORTEN SIMONSEN 



Preimttiunization of donors 



The immune C3H donors have all been injected intraperi- 

 toneally with spleen cells from F^ hybrids which were isogenic 

 with the prospective test litter. 



There are two reasons for not using foreign pure-strain cells as 

 antigen source : (i) they might possibly survive in the prospective 

 C3H donor's spleen long enough to give a graft-versus-host 

 reaction of their own, after injection into the infantile Fj hybrid; 

 and (2) they might also produce a graft-versus-host reaction in the 

 C3H mouse, which fact, in consequence, might interfere with the 

 process of immunization, especially in hyperimmunized mice 

 (cf. Brent and Medawar, 1962). The doses used for immunization 

 have been ^-i hybrid spleen per injection. Repeated injections 

 have always been spaced at least i week apart; usually 3 weeks 

 have elapsed between the fu:st and the second. 



Results 

 Factor of ittimunization in response to ST/ A antigen 



This will be mentioned first because a full account of the data 

 (Table I) together with the graphical distribution of the mean 

 values of the spleen indices (Fig. i) serve to illustrate the meaning 

 of the term. Factor of Immunization (F.I.). 



The bottom of Table I gives the sums and means of the spleen 

 indices for the 6 points of titration, 3 points representing normal 

 (N), and 3 points immune (/) donors. 



The means are plotted in Fig. i against the log doses employed, 

 and 2 straight lines have been drawn in parallel to fit the data. The 

 slope and position of these lines have been determined as follows. 



The common slope is based on the assumption that all differences 

 in observed slopes from one test litter to another as well as 

 between normal and immune curves within the same litter are 

 all due to sampling error. Hence, they are all more or less 

 imperfect terrestrial copies of the true platonic slope for this 



