In the past few decades, scientists' understanding of microchimerism has grown, thanks to technological advances. And the implications are wild.
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.
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?
Hannah Osborne: What first got you interested in microchimerism? How did your personal understanding change over the course of writing "Hidden Guests"?
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.
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.
LB: Maybe before speaking about the hindrances, just saying that the first time we discovered cells from other individuals within individuals was a century ago. The first one was in 1893, and it was a German pathologist who discovered cells from fetal origin in lungs from women who had died during pregnancy. Then it took a century to understand that this phenomenon happened to everyone and that it is of importance.
Maybe a third hindrance is that at the beginning of this field — I mean the new beginning in the '90s — they were mostly women working in this field. They told me that male colleagues often said, "Well, that's woman's stuff, you know; you're looking for some connection with your babies, but it's not really relevant," which is wrong, of course, because even men are affected by this phenomenon. They stayed nine months in the womb like us.
It's because it's such a new field and you have such open questions, there's room for many interpretations, many hypotheses.
LB: I guess it'd be the story of Lydia Fairchild because she nearly lost custody of her children because of microchimerism. She was 26 and living in the U.S., and she applied for a welfare benefit to help her raise her kids. In the U.S., you need to undergo a maternity test for that, and her DNA test ends up showing that she could not be the mother of her kids. Of course she tried to argue; she had photos; she had testimonies. But she was always told, "DNA is 100% foolproof; DNA doesn't lie," which implied that she was lying. But she was not, so she called several lawyers. But at the beginning, they were all saying, "No, DNA doesn't lie."
In fact, she was harboring cells from her vanished sister inside her. Most of her eggs carried her sister's genetic signature, which means that the mother of her kids never lived. Isn't it amazing? That's maybe one of the stories that blew my mind.
LB: The truth is that I think we don't really know. If you have only some microchimeric cells, among thousands of your own cells, then it won't change the reliability of the DNA test. But in some cases, these microchimeric cells can make up a significant part of an organ and sometimes even an entire organ. This proportion might depend on when you received those microchimeric cells. For instance, if you received those cells like Lydia Fairchild at the very, very beginning of your development, they might end up making up the entire organ where they landed first. But probably, we don't know which percentage of us start our in utero life with an evanescent embryo.
An illustration of a human embryo (Image credit: RUSLANAS BARANAUSKAS/SCIENCE PHOTO LIBRARY via Getty Images)But there's another example in my book that took place in Alaska in 2004. After a rape, the forensic investigators collected the semen from the scene, and they discovered that the DNA in this semen matched with a man who was already in the database. But this guy was behind bars. He was in jail, the day of the rape, so he couldn't be the rapist.
I guess there are some cases where microchimeric cells can really change the result of a DNA test. But for now, it's not really studied. I've been contacted by some forensic investigators to learn more about microchimerism, so maybe things will change.
LB: Next time you're at a dinner, just try to explain microchimerism. You're a science journalist, right? Well, usually when you tell the story you're working on, people are saying, "Oh, yeah, OK." And they just turn to another conversation. If you try mentioning microchimerism in your next dinner, then you'll see. Everyone will try to make some hypothesis, some connection; some will be moved that it's possible to keep some cells from an unborn child or a lost child. Some will be disgusted by the idea that they keep cells from their mother, like having their mother on their shoulder all the time. It's really the first time as a science journalist I experienced such resonance with a story I was working on among people around me, people who are really not into science.
It's because it's such a new field and you have such open questions, there's room for many interpretations, many hypotheses. I mean, that's fine; that's how science works. You should always have hypotheses and try to answer your hypothesis with new experiments. But it's true that sometimes I didn't know how to handle all those ideas at the beginning. Then I realized it was like a window open on the intimate — their fears and their desires — and it reveals what they really think about their connection to the world they are in. From a sociological point of view, it would be really interesting to study that.
LB: Many scientists told me that until 2021 or something like that, there was this hypothesis that those cells could come from another generation than the mother. We knew that the mother would give cells to the fetus and the fetus could give cells to the mother, but then people ask, because the mother can still carry cells from her mother, maybe she could pass those cells to the fetus. Until the 2020s, scientists were thinking, "Well that's a crazy hypothesis; that can't happen." There could be only a few cells that won't go through the placenta and integrate with the new fetus — until one scientist proved that a newborn can carry cells from the grandma.
This hypothesis is that we could inherit cells from our sexual partner. I thought it was not possible because I thought the semen was full of spermatozoa, that they are only like half cells having only half chromosomes, so they cannot survive. But actually in semen, you have a lot more cells than spermatozoids — you have immune cells, for example.
We should be careful about those interpretations. But I think scientists are not the ones who should care about this interpretation; otherwise, they would prevent themselves from doing their research. They should open all the doors with all kinds of hypotheses. If we do it in a scientific way, meaning if we try to answer a hypothesis with good experiments, then I think everything is fine. You will never prevent people around you from using this information to distort the facts and tell another story. That's how life goes.
Related storiesHO: What do you think the most promising use for microchimerism research will be over the next 10 to 20 years? How do you see it unfolding?
Maybe also in the transplantation field; maybe we can think about using the knowledge of how microchimeric cells can induce tolerance to avoid [immunosuppressive] drugs. When you receive the transplant, you need to take drugs [that suppress the immune system] for the rest of your life, and those drugs are like really strong drugs. If we can use microchimeric cells before the transplant to allow better tolerance, then we could either not use any drugs or maybe use less drugs. That would be really interesting, too.
greystonebooks Hidden Guests: Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity
What if some of your cells were not your own? What if they once belonged to someone else?
Part mind-bending medical mystery — part cutting-edge science — Hidden Guests uncovers the astonishing phenomenon of microchimerism: the presence of foreign cells inside our own bodies. The incredible story of how those cells got there — and what they do once they arrive — might change everything we know about the immune system, lineage, and identity.
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