In 1996, Lee Nelson proposed that microchimerism might make some mothers sick. With half their genetic material coming from their father, fetal cells might be a confusing mix of the foreign and the familiar. Nelson speculated that being exposed to fetal cells for years on end could lead a woman’s immune system to attack her own tissues. That confusion might be the reason that women are more vulnerable to autoimmune diseases such as arthritis and scleroderma. To test this possibility, Nelson and Bianchi collaborated on an experiment. They picked out thrity-three mothers of sons, sixteen of whom were healthy and seventeen of whom suffered from scleroderma. Nelson and Bianchi found that the women with scleroderma had far more fetal cells from their sons than did the healthy women.
But maybe this can be good?
It’s also possible that being a chimera can be good for your health. Bianchi’s first clue that chimerism might have an upside came in the late 1990s, when she was searching for fetal cells in various organs. She discovered a mother’s thyroid gland packed with fetal cells carrying Y chromosomes. Her gland was badly damaged by goiter, and yet it still managed to secrete normal levels of thyroid hormones. The evidence pointed to a startling conclusion: A fetal cell from her son had wended its way through her body to her diseased thyroid gland. It had sensed the damage there and responded by multiplying into new thyroid cells, regenerating the gland.
What about getting genes from a baby that is not genetically one’s own?
As chimerism rises out of the freak category, it also raises unexpected ethical questions. Somewhere around a thousand children a year are born to surrogate mothers in the United States alone. As Ruth Fischbach and John Loike, two bioethicists at Columbia University, have observed, the rules for surrogacy are based on an old-fashioned notion of pregnancy. They treat people as bundles of genes. As a society, we are comfortable with a woman nourishing another couple’s embryo and then parting ways with it, because she does not share the hereditary bond that a biological mother would. If the pregnancy goes smoothly, the surrogate mother is supposed to leave the experience no different than before the procedure. But Fischbach and Loike observed that a surrogate mother and a baby may end up connected in the most profound way possible. Cells from the fetus may embed themselves throughout her body, perhaps for life. And she may bequeath some of her cells to the child. This is not merely a thought experiment. In 2009, researchers at Harvard did a study on eleven surrogate mothers who carried boys but who never had sons of their own. After the women gave birth, the scientists found Y chromosomes in the bloodstreams of five of them.