Journalist and author Lise Barnéoud talks to Live Science about her book "Hidden Guests," which dives into the field of microchimerism and how it's transforming our knowledge of genetics, immunology and evolution.
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Over a century ago, German pathologist Georg Schmorl documented a strange phenomenon: cells from fetuses that had moved into the mother during pregnancy. He had discovered microchimerism — where the cells from one individual move into another and set up shop.
In the past few decades, scientists' understanding of microchimerism has grown, thanks to technological advances. And the implications are wild.
Cells transferred during pregnancy could mean you have cells from your grandmother living inside you. A twin can be "absorbed" within the womb, leaving a genetic trace of the lost sibling. And after a bone marrow transplant, a man ended up with the donor's DNA in his blood and semen. These are just a few examples of how microchimerism opens a new way to understand immunology, genetics and evolution.
In her book "Hidden Guests: Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity" (Greystone Books, 2025), author and journalist Lise Barnéoud explores how the field emerged and developed as scientists grappled with understanding what was going on, and why — as well as the far-reaching implications of these minuscule interlopers.
"Hidden Guests" has been short-listed for the 2026 Royal Society Trivedi Science Book Prize.
Excerpt from "Hidden Guests": 'It doesn't lie. So who are you?': What happens when DNA tests show a woman is not the mother of the child she gave birth to?
Lise Barnéoud is a freelance science journalist who regularly contributes to Mediapart, Science et Vie, Le Monde. In addition to Hidden Guests, she is the author of two books about vaccines (not yet translated). In 2017 she won French Science Writer of the Year and was nominated for European Science Writer of the Year. Lise lives in France.
Live Science spoke with Barnéoud about the book, why the idea of microchimerism resonates so deeply, and where the field could be headed next. "Hidden Guests" has been short-listed for the 2026 Royal Society Trivedi Science Book Prize.
Hannah Osborne: What first got you interested in microchimerism? How did your personal understanding change over the course of writing "Hidden Guests"?
Lise Barnéoud: The first time I heard about it, I was writing my previous book on vaccinations, and I had a chapter on our immunity, how our immune system works. I wanted to explain what we got wrong about how our immunity works. We are still learning — [we used to have this idea] that our immune system is kind of an army patrolling in our body and expelling any foreign elements. That's the image we have of our immune system. I wanted to tell another story, which is more accurate because we know now that it's not working like that.
I was looking for counterexamples, and, of course, one very famous counterexample is pregnancy. We tolerate the fetus that is half different than us. Another well-known counterexample is microbiota. Inside our body, we carry microbial cells as well as human cells — so cells coming from bacteria, viruses, fungi. We don't expel them; we need them. The third example I found was microchimerism.
The more I read about it, the more I was interested in it. It was challenging so many things. I've learned that the placenta is not that watertight barrier that we often think of, that the immune system is not as intolerant as we often think, and we can inherit cells from our kids like an inverted inheritance. All those elements popped up within my first reading, and I thought, "OK, I really need to dig more."
For your second question, I think the biggest change is that I realized those cells are not only kind of inactive tourists, just homeless drifters passively traveling in our body. They are really active. They can multiply, they can produce protein, they communicate with other cells, and they really influence our physiology, for better or for worse.
