DISCUSSION 



Medawar: With regard to the effect of repeated pregnancies, Dr. 

 Sparrow and I did quite a number of experiments of this kind a few 

 years ago (Medawar, P. B., and Sparrow, E. M. [1956]- J. Etidocr. 14, 

 240) and found that the results, as I am sure Dr. Barrett would confirm, 

 do vary considerably with the strain combination that is used. In the 

 direction A to CBA, passage through repeated homospecific or hetero- 

 specific pregnancies does not affect the survival time of skin homo- 

 grafts; but in grafting the other way round, from CBA to A, then 

 passage through repeated heterospecific pregnancies does significantly 

 prolong the life of skin homografts. It is also worth remembering that 

 there is also some prolongation in mice which have been through 

 repeated homospecific pregnancies — an essential control — and also in 

 elderly virgin female mice of the same age. 



Dr. Hasek, you gave three possible explanations why the foetus 

 might not immunize the mother. I should like to suggest a fourth: 

 that antigens do pass from the foetus to the mother by a route which is 

 immunologically ineffective, at least as far as sensitization is concerned. 

 Do you think this is a possibility ? 



Hasek: I agree that this might be a fourth explanation. However, 

 the intravenous route of antigen administration, which is likely to be 

 involved in the mother-foetus relationship, can by no means be con- 

 sidered as ^generally ineffective form of sensitization even though this is 

 the case as far as intravenous administration of epidermal cells is 

 concerned. 



Medawar: Oh no, certainly not; only when the antigen is in a soluble 

 form. 



Krohn: I can certainly confirm that the placental tissue is capable of 

 stimulating an immune response, but I have done it only in the direct 

 transfer of CBA placental tissue to an A mouse; there the subsequent 

 test grafts have broken down as vigorously as if the animal had been 

 immunized with any other tissue. 



I wonder whether there aren't some other possible explanations for 

 the difference between the response of the male and female strains in 

 your experiments. One might conceivably resuscitate the discarded 



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