Researchers at the Medical College of Georgia have found a chemical made by fetal cells that protects against a mother's immune system.

From CNN Medical Correspondent Dan Rutz

AUGUSTA, Georgia (CNN) -- Researchers at the Medical College of Georgia appear to have solved one of the great mysteries of biology -- how pregnancies are able to progress without the mother's immune system rejecting her fetus.

"What we've discovered is that the fetus comes in with a rather surprising mechanism that nobody anticipated to protect itself from the mother's immune system," said researcher Andrew Mellor.

Genetically, a mother's children are only half hers. The father's half is, technically speaking, foreign tissue which the body's immune system would ordinarily reject.

That's why organ transplant recipients need strong drugs to protect their new kidney, liver or heart from being destroyed by natural immunity.

The college's scientists zeroed in on a chemical made by the fetal cells which, in a chain reaction, selectively shuts down that part of the mother's immune response that would otherwise harm the fetus.

"In the placenta, this mechanism is a natural mechanism for persuading the immune system to tolerate a foreign tissue that it would otherwise like to reject," said researcher Dr. David Munn.

Using mice, the researchers proved the chemical is essential to a successful pregnancy. When it is blocked, without exception, the mother quickly miscarries as the immune system tears up the fine fabric of normal cell development in a matter of days.

"The essentially all or none nature of the outcome was reassuring that this is real biology and not some non-specific or accidental effect," Munn said.

The finding may help explain recurrent miscarriages in humans, as well as diseases such as juvenile diabetes or lupus -- diseases in which the immune system turns against the body.