DISCUSSION 131 



As to this barrier between the mother and foetus, this bothers me. 

 Dr. K. Benirschke, of Dartmouth, who has done a great deal of work 

 on the placenta, spoke of the human placenta as having a very tight 

 barrier which I think he ascribed to a fibrinous material. Others have 

 talked about other types of barriers. But we are confronted with the 

 fact that, as Dr. Hasek says, the incidence of Rh disease is less than is to 

 be expected statistically. We think our tolerance phenomena might be 

 a partial explanation for this. Still the antigen does get across, and what- 

 ever channel it traverses is an effective one; the mother does become 

 effectively immunized. 



Now there is one other difficulty in my mind. Dr. H. Meier (1961. 

 Experientia 16, 215) reported a few months ago that one can distin- 

 guish, by histochemical reaction, the erythrocyte of the mother from 

 the erythrocyte of the hybrid. He reported that the blood of hybrid 

 foetuses contained a mixture of hybrid and maternal erythrocytes. If 

 this can be substantiated, whatever the placental barrier is, in this com- 

 bination in the mouse whole erythrocytes can cross and form a tem- 

 porary chimera in the hybrid. 



Hasek: I should Hke to come back to Prof Krohn's comment. Our 

 experiments are fully in agreement with antigenic activity of placental 

 tissue, but this apphes only to maternal antigens whereas those of the 

 paternal strain are missing when hybrid placentas are tested. The re- 

 moval of blood from placentas in our experiments was certainly not 

 complete. As to the two alternative explanations suggested by Prof. 

 Krohn concerning the origin of placenta: if M. Galton's (i960. Lancet, 

 I, 495) preliminary finding of DNA content of trophoblast cells were 

 confirmed it would be pretty direct evidence in favour of the second, 

 more compHcated hypothesis. 



Barrett: We washed the placenta until no further blood could be seen 

 coming out into the wash water. Although there probably was some 

 blood left on the placenta, it was certainly a quite inadequate dose to 

 have produced the immunity that we saw in the strain combination 

 that we used. 



One further point related to what Dr. Krohn said: our system was 

 different from his; in our case any non-paternal material injected with 

 the placenta was of maternal origin injected into the maternal strain and 

 therefore could not have contributed to the effect we saw. It is essential 



