A study has found that foetuses actively protect themselves against attack by the mother's immune defences, a finding that could help to explain why some women suffer repeated miscarriages.
Peter Medawar, an eminent British geneticist, first highlighted the mystery of why pregnant women should not reject their babies in 1953. He said a foetus shares half its genes with its father and should therefore become a victim of tissue rejection by the mother's immune system, which is primed to attack ``foreign'' cells.
Medawar suggested three possible explanations. First, that there is some sort of anatomical separation of mother and baby in the womb; secondly, that the tissues of the foetus are too immature to stimulate the mother's immune system or, lastly, that the mother is somehow tolerant of her unborn baby's ``foreign'' genes.
The latest study into the problem published in the journal Science has, however, found evidence to support a fourth explanation - the embryo actively shuts down the mother's immune defences.
David Munn and colleagues from the University of Georgia found that foetuses produce an enzyme called IDO that destroys tryptophan, an amino acid that acts as an important stimulant for the attacking T-cells of the mother's immune system.
The researchers tested the idea on mice. Professor Munn concludes that the production of IDO in the placenta by the foetus is the mechanism that ensures its survival against the potentially lethal effects of its own mother's immune system.
The scientists suggest that new drugs could be developed to work in a similar way to IDO, and thereby dampen down the immune system of those women who have a history of miscarriages.