Foetal cells that survive in the mother after pregnancy may trigger diseases in the woman many years later.
The discovery that these cells can persist in the mother's bloodstream may provide a reason why women are three times more likely than men to suffer from auto-immune diseases.
These are a family of about 100 different disorders which all share a common feature. They are caused when the body's own immune system which protects against bacteria and viruses suddenly turns against healthy tissues.
Certain tissues are destroyed, leading to diseases such as rheumatoid arthritis, lupus, multiple sclerosis and a progressive and often fatal hardening of tissues called scleroderma.
The discovery that the foetus might be an unexpected enemy from within was presented yesterday at the American Association for the Advancement of Science annual meeting in San Francisco. The work was done by Dr Lee Nelson and colleagues of the University of Washington in Seattle.
Women on average are three times more likely than men to get an auto-immune disease and for certain diseases 10 times more likely.
Dr Nelson's theory was that foetal cells which survived for decades in the woman's bloodstream after pregnancy somehow affected the woman's immune system over time and caused it to malfunction and cause damage.
She led a research team looking for "foreign" DNA in blood samples taken from healthy women and from women with scleroderma.
They searched for evidence of male DNA, signs of the male Y chromosome. They found what they were looking for. Women with scleroderma had 11.1 foetal cells per million maternal cells tested. Less than one foetal cell in a million was discovered in those without the disease.
"Pregnancy is mini-gene transfer," Dr Nelson said. "Those genes that are transferred from the child can in-graft and persist, probably indefinitely." Even 30 years after having a child these foreign cells can be discovered in women who had children.
"It turns out that there is cell transfer in both directions," she said. The mother's cells can be found in her sons and there is now also evidence that cells from the grandmother passed to her daughter can reach the bloodstream of the granddaughter.
She is now trying to make the connection between these foreign cells and the high rate of auto-immune diseases in women.
