Studies conducted upon inbred strains of mice, hamsters and rats have shown that following the interstrain matings the now familiar covert reactivity of pregnant females to the alloantigens of their conceptuses may benefit the latter in two ways; firstly, it exerts a significant influence upon placental weight, and indirectly upon the birth weight of the fetus-allogeneic placentas tending to be heavier than syngeneic placentas, and mothers specifically presensitized against alien paternal tissue antigens gestate fetuses with heavier placentas than normal females. Specifically tolerant mothers, on the other hand, produce smaller, F1 hybrid, fetoplacental (fp) units. The classic notion that the disparity between the birth weights of F1 hybrid and homozygous offspring is due to hybrid vigor has been challenged by the finding that DA and (DA times F1)F1 hybrid blastocysts transferred to the uteri of genetically tolerant (DA times F1)F1 hybrid rats produce fp units of similar weight Maternal immunological reactivity against the fetus qua allograft may make a significant contribution here. Additional support for the premise that maternal reactivity against fetal alloantigens in some way promotes the growth of the fp unit was afforded by the finding that excision of the para-aortic lymph nodes (which drain the uterine horns) from females before interstrain matings resulted in smaller fp units than in females subjected to sham operations. The finding with one rat strain combination that passive immunization of females with serum against their F1 hybrid conceptuses promoted the growth of the latter suggests that a humoral rather than a cellular immunity may be involved. Secondly, in the three species studied, it was observed that genetic disparity between a conceptus and its mother significantly improved its chances of implantation and development to term.

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