124 M. HASEK ET AL. 



as the occurrence of immunity in the mother is concerned, cases 

 of Rh incompatibihty are currently known, but they are much 

 rarer than the theoretical immunological presumptions would 

 suggest. Even if transfer of cellular elements between mother and 

 foetus does not seem to be extremely rare under normal conditions 

 (Zipursky et ah, 1959), its possible immunological consequences 

 are not very frequent. 



If the placental transfer of cellular elements were equally 

 frequent in both directions, the immunological consequences on 

 the part of the mother should be opposite to those on the part of 

 the offspring, reflecting the critical role of the stage of immuno- 

 logical maturity of mother and foetus for the reaction pathway 

 which a given antigenic stimulus can initiate. From this point of 

 view, therefore, it is rather surprising that interstrain or intra- 

 strain parity has been demonstrated to induce a tolerance-Hke 

 state to the male-strain or male-sex antigens. 



Breyere and Barrett (1960^) described the effect of previous mat- 

 ings of BALB/c female mice to DBA/ 2 males on their resistance 

 to tumour of DBA/ 2 origin. The tumour grew better in these 

 post-partum females than in virgins or females parous by BALB/c 

 males indicating a change in the natural immunogenetic resistance 

 in the direction of some sort of "tolerance". Later on, these 

 authors (i96o/>) extended their observations to normal tissue 

 (skin homografts) towards which a partial tolerance resulted in 

 females from intrastrain matings. Within certain limits graft 

 survival seemed to depend on the number of previous pregnancies, 

 but after the third pregnancy the maximum effect available in the 

 given system was apparently achieved. 



This second result was in accord with that of Prehn (i960) 

 who was able to produce a fairly high percentage of isologous 

 male skin grafts in CsyBL/An female mice. The degree of 

 tolerance was roughly proportional to the number of previous 

 pregnancies, but a partial tolerance could also be induced by 

 matings with sterile males. Since the basis of the male sterihty 



