MOTHER-FOETUS RELATIONSHIP I27 



factors than insemination can scarcely be held responsible for this 

 sort of immunological inhibition but the mechanism of its effect 

 is to be explained. According to some speculations, immunity 

 was expected to result sometimes from repeated inseminations 

 rather than the opposite modification of immune response but 

 there is no critical evidence for it so far. 



The fact that repeated pregnancies decrease the reactivity of 

 females seems to indicate that perhaps another safety mechanism 

 exists in case of failure of the barrier function of placenta. Instead 

 of immunity, which might be expected, there occurs in parous 

 females an inhibition of reactivity, specific towards antigens of the 

 breed male. It seems probable that the compatibility between 

 mother and foetus is the outcome of more than one mechanism, 

 among which the antigenic inferiority of the syncytiotropho- 

 blast and its function as a semipermeable membrane could play a 

 role. However, the occurrence of other immunological pheno- 

 mena shows that in the relationship between mother and foetus 

 the mechanisms inducing compatibility predominate, rather than 

 those inducing immunity. The question remains of whether an 

 absolute mother-foetus compatibihty is required for no immuno- 

 logical comphcations of pregnancy to occur, or whether such 

 comphcations might not be due to a critical threshold of incom- 

 patibility being reached. 



Stmiinary 



Except in intra-inbred mating, the foetus with the foetal part 

 of the placenta may be regarded as a homograft. The foetus 

 from inter-inbred mating does not induce an immune reaction 

 under natural anatomical conditions but is capable of doing so after 

 a heterotopic transfer into individuals of the female strain; 

 however, the foetal part of the placenta is antigenically inferior 

 in the sense that it does not immunize individuals of the female 

 strain under conditions in which the actual tissues of the foetus 

 would have done so. 



