“Hidden Guests
By Lise Barneoud
Greystone, 200 pages, $27.95
In 2004, a crime lab in Alaska used DNA technology to track down the perpetrator in a sexual assault. Technicians analyzed semen collected from the victim and found a match in their database to a man whose DNA had been sampled when he'd been previously arrested. It seemed to be a straightforward case, except for one snag: The man the database found was in prison when the rape occurred. He couldn't have committed it.
The baffled cops investigated and discovered that the man in question had received a bone-marrow transplant from his brother years earlier. Further biological testing revealed that the transplanted cells had somehow replicated throughout the man's body. As a result, when samples were collected after the man was jailed, his brother's DNA had ended up in the database. After further investigation, the police arrested the brother.
However bizarre it may be, this case of microchimerism -- the persistence of cells from one person in the body of another -- is not unique.
Indeed, as the French science journalist Lise Barneoud explains in her fascinating book, "Hidden Guests: Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity," the phenomenon isn't even rare.
It happens all the time during pregnancies.
Although scientists once believed that the placenta was a tight, impermeable barrier separating mother and child, that's not true. Fetal cells regularly tunnel through it into the mother and get incorporated into her body. In fact, all mothers likely carry cellular traces of their children. As Ms. Barneoud writes, DNA normally travels only from parent to offspring, but microchimeric cells "are climbing up the family tree, traveling back in time."
Conversely, maternal cells circulating in the pregnant mother's bloodstream commonly slip through into the fetus and get incorporated into its body. And not only maternal cells. Cells from older biological siblings -- or even the mother's relatives -- can end up inside the fetus.
Women have been found to carry cells left behind by male sexual partners, years after an encounter. These microchimeric cells persist for decades and can embed themselves in any organ, including the woman's brain.
As Ms. Barneoud notes, some people find microchimerism comforting, while others feel creeped out. (As one geneticist in the book quips, "you think your mother is always looking over your shoulder? She may be in your shoulder.") The big question is: What are these interloper cells doing inside us?
Scientists are still sorting out the answer, but it's clear that these kinds of cells aren't merely passive lurkers. In the mid-1990s, scientists began to link microchimerism to several medical disorders. Pregnant women with higher rates of microchimeric cells tend to have higher rates of complications and miscarriages. The cells also seemingly play a role in autoimmune disorders, in which the immune system mysteriously attacks the body. When scientists studied women with such diseases, they often found fetal cells in the affected organs: the skin in scleroderma, the liver in biliary cirrhosis, the joints in rheumatoid arthritis and lupus. The researchers suspected that the immune system might be attacking these intruders and causing collateral tissue damage. It seemed to be a huge breakthrough.
Except, as Ms. Barneoud points out, this theory has flaws. When scientists looked at organs unaffected by disease, they found fetal cells in those, too. Moreover, fetal cells can migrate throughout the body, especially in response to inflammation and other signs of biological distress. So it's possible that the fetal cells in damaged organs were there to provide aid. As Ms. Barneoud puts it, blaming these cells for being present in damaged organs "would be like accusing firefighters of starting forest fires because they show up every time a blaze rages through the woods."
When researchers looked more closely, they discovered that microchimeric cells have other beneficial effects. Fetal cells can seemingly weave themselves into a mother's damaged tissue and repair it.
Babies also benefit from maternal microchimeric cells, which help train the immune system to fight off infections. A 2007 study documented a case of a mother's cells producing insulin for her diabetic child whose own pancreas could not do so. In the case of organ transplants, the mingling of donor and host cells seems to lower the chances of rejection. Someday, the author suggests, transplant recipients could be given donor cells before surgery to raise the odds of success.
As a book, "Hidden Guests," ably translated from French by Bronwyn Haslam, is more lyrical than narrative. Microchimeric cells can wreak havoc with genetic paternity tests and DNA analysis in criminal cases. But Ms. Barneoud does little more than outline such issues. Instead, she ruminates on how microchimerism might change our understanding of ourselves. Ms. Barneoud writes that these discoveries have led her to reject the idea of an autonomous self: "There is no logic to 'me' versus 'them' because the two are intertwined, blurring the limits of our individuality." She compares the human body to a coral reef, where animals, plants, and microscopic organisms "cohabitate as one huge and beautiful chimeric metaorganism."
Ms. Barneoud is sharpest when she examines the metaphors that immunologists and others have used to describe microchimeric cells, which have been characterized as invaders, migrants and colonizers. Like it or not, those terms have value judgments attached to them that can be hard to square with the mounting evidence that microchimeric cells might do more good than harm.
The title of "Hidden Guests" is Ms. Barneoud's attempt to rehabilitate microchimeric cells with a new metaphor. But the most memorable characterization comes from an immunologist she cites, whose patients sometimes ask whether the foreign cells inside them are good or bad. His answer is yes. "Demons are but fallen angels," he muses, and "you can never predict which way a cell will go."
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Mr. Kean is the author of "Dinner With King Tut: How Rogue Archaeologists Are Recreating the Sights, Sounds, Smells, and Tastes of Lost Civilizations."” [1]
1. She Has Her Mother's Cells. Kean, Sam. Wall Street Journal, Eastern edition; New York, N.Y.. 05 Nov 2025: A15.
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