Instant Genius - How our bodies are host to mysterious cells that came from other people

Episode Date: October 19, 2025

The commonly held story goes that we all begin life as a single cell in our mother’s womb and go on from there to develop into fully formed adult human beings composed of cells of our own creation. However, recent research is showing that this isn’t quite the full picture. Not all of the cells in our bodies come from this single starting point. We are, in fact, all host to cells that have migrated from the bodies of other people – our mothers, grandmothers, siblings and even, in some cases, complete strangers. In this episode, we’re joined by science writer Lise Barnéoud to talk about her latest book Hidden Guests - Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity. She tells us how mothers and babies share cells across the placenta during pregnancy, how these cells make their homes within our organs and persist for our entire lives, and how this mysterious process led to a woman giving birth to the genetic children of her unborn twin sister. Learn more about your ad choices. Visit podcastchoices.com/adchoices

No one goes to Hank's for his spreadsheets. They go for a darn good pizza. Lately, though, the shop's been quiet. So Hank decides to bring back the $1 slice. He asks Copilot in Microsoft Excel to look at his sales and costs to help him see if he can afford it. Co-pilot shows Hank where the money's going and which little extras make the dollar slice work.

Now, Hank has a line out the door. Hank makes the pizza. Co-Pilot handles the spreadsheets. Learn more at M365Copilot.com slash work. When you need to build up your team to handle the growing chaos at work, use Indeed sponsored jobs. It gives your job posts the boost it needs to be seen and helps reach people with the right skills, certifications, and more. Spend less time searching and more time actually interviewing candidates who check all your boxes. Listeners of this show will get a $75-sponsored

job credit at Indeed.com slash podcast. That's Indeed.com slash podcast. Terms and conditions apply. Need a hiring hero? This is a job for Indeed sponsored jobs. Introducing the new best skin ever ultra-slim precision concealer from Sephora Collection. It's full coverage with a matte finish and perfect for any look, whether you're building it up for a full glam moment or targeting correction for a more natural vibe. At only $12, it's great for affordable touch-ups on the go. Get this new must-have concealer at Sephora or at Sephora.com today. This podcast is sponsored by Name, Audio and Focal.

Streaming has made music more accessible than ever, but true listening is about more than ease. It's about quality. British audio experts name audio, alongside French acoustic specialist Focal, combine handcrafted tradition with cutting-edge innovation and high-end materials, delivering digital precision with analogue warmth.

So you can experience exceptional sound at home. Music just as the artist intended. visit name audio.com to learn more. Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. Every Monday and Friday, you'll hear a world-leading scientist and experts talking about the most fascinating ideas in the science and technology today. I'm Jason Goodyear, commissioning editor, a BBC science focus. The commonly held story goes that we all begin life as a single cell in our mother's womb

and go on from there to develop into fully formed adult human beings composed of cells of our own creation. However, recent research is showing that this isn't quite the full picture. Not all of the cells in our bodies come from this single starting point. We are, in fact, all host to cells that have migrated from the bodies of other people, our mothers, grandmothers, siblings, and even in some cases, complete strangers. In this episode, we're joined by science writer Lee's Bonnude to talk about her latest book, Hidden Guests, Migrating Cells and How the New science of microchimerism is redefining human identity. She tells us how mothers and babies share cells across the placenta during pregnancy, how these cells make their homes within our organs,

and persist for our entire lives, and how this mysterious process led to a woman giving birth to the genetic children of her unborn twin sister. So welcome to the podcast. Thanks so much for joining us. Thank you for your invitation. So today we're talking about your latest book, Hidden guests, migrating cells and how the new science of microchimerism is redefining human identity. So let's start with the sort of big question then. I think most people listening will assume that we're simply a product of our genes. But over the last few decades, we've also learned that we're also home to a colony of microbes that we live in tandem with that affects us in profound ways too. But in your book, you write about something that's another layer of this called microchymic.

So first off, can you give us a brief description of what exactly that is? Yeah, exactly. So microbiota, we discover with microbiota that roughly half of our cells that compose us are microbial cells, either bacteria, viruses, fungi, but microchimerism is adding a new layer. Even this human half of us does not solely come from our own cells, some of those human cells that compose our body are coming from other individuals, from other humans. So we call them microchimeric cells, micro because they are not so frequent, it will depend on the individuals and it will also depend on the organs within an individual.

But usually we will find once microchimeric cells for 10 or 100,000 other cells, and we use the term chimera in reference to the monster from the Greek mythology, which is both goat, snake and lions. And we use that term for organism that consisting of tissue from diverse genetic constitution. So that's the big picture of microchimerism. Yeah, so let's have a look at how some of this occurs. So I think the best place to start really is with fertilization. You talk about this a lot in your book.

And this is something that I didn't know. It's a surprisingly sort of violent process and one that got started many years ago, thanks to the action of a virus of all things. Yeah, that's also something I discover during my book. So a few days after the fertilization, you have this button of cells, the blastocytes. And cells from the outer layer of this blastocyst are called trophoblast, and those cells will fuse together, and they will become giant cells with multiple nuclei. And they will have a very aggressive behavior. They will attack the maternal cells, and they will, you know, bring the blastocysts within the uterine lining until they reach the blood vessels.

a bit like roots looking for water and nutrients, but except that in this case, these roots carry the seeds inside the mucosa. And some of the researchers think that there is a parallel between this behavior of the cells and cancer cells because they also evade immune system and they also recruit blood vessels. And it's true that this mechanism is due to a virus

or ancestor encountered tens of millions years ago, actually around 8% of our genomes are made of viral DNA that we encountered during our lives. And most of them are silent, but some of them are playing a role. And this specific gene called H-E-R-V-W is the one that allows trophoblast to invade the uterine. If you turn it off, the embryo cannot develop. So in a certain way, we can say that if our ancestor hadn't encountered this specific virus,

maybe we will still be laying eggs, and we won't be concerned by the microchimerism phenomenon. Yeah, so I think one thing that a lot of people would think when somebody is pregnant with a fetus, is that it's all sort of one-way traffic from the mother. to the fetus. But you say that cells from the unborn child actually travel in to the mother's bloodstream and we can detect these throughout their entire lives.

I mean, that's mind-blowing. Mm-hmm. Yeah, so the travel of those cells are going both ways. So from the mother organism towards the fetal organism and from the fetal organism toward the mother organism, Those cells are crossing the membrane of the placenta and they are traveling in the other body. And that's true that we shouldn't consider these cells as tourists visiting new places for some days. Actually, they will settle in this or that organ and they will stay there probably for the rest of our life because they have this characteristics of stem cells, meaning they can renew themselves.

So they will divide into daughter cells, which will also divide in daughter cells. And they will stay there like a little cell population. The only difference between this population of cells comparing to her cells is that they are carrying a different genetic signature. So what sort of effects can that have on the mother's body? That's one of the big questions that animates researchers today. At the beginning, they were mainly focused on negative aspects. They realized that in some cases, these cells can provoke inflammation or even attack or organs.

And that's why there is this hypothesis that those cells can be linked with some autoimmune disease, which would in that case not be autoimmune but alloimmune because those cells are not really our cells. So, but that was the initial food, but soon researchers realized that these microchimeric cells have also a lot of beneficial roles. And the most documented one is that those cells can regenerate organ. They can repair organ because they can not only renew themselves, but they can also specialize in every tissue they settle in. For instance, if I received cells from my first son that is in my body and these cells settle in my lung, then these cells will become specialized in lung cells. They will follow the specification in the organ they are settled in. And in that way, they can repair our organ.

For instance, we have some really nice experiment that show if you have a cut in your body. skin, cells from the damaged area will produce some inflammatory molecules called chemokine. Our immune cells are sensitive to these molecules. That's why they are coming in this specific area to try to repair the wound. But our microchimeric cells have a lot more receptors to these molecules and they can kind of hear the damage. signal from far away. And that's why they will follow the concentration of this molecule within our body until they reach the cells that are producing this inflammatory molecule.

And while they will be there, they will both specialize in skin cells, but they will also create new blood vessels. They will also transform in collagen. So they will really help to regenerate, to repair the wound. And that's one of the most documented role now that we have on microchymic cells from fetal origin. Yeah, so in the book, you run through several really fascinating case studies, one of which is the case of Karen Kegan, who actually had children whose DNA did not match

with hers. I mean, on first glance, that seems impossible. So what happened there? Yeah, that's a really fascinating case. So Karen Kagan, she had three sons and two sons did not carry half of her DNA. That's what's expected when you are a relation mother and son. But what we realized that she was not sharing 50% of DNA, but she was sharing 25% of DNA. So in a genetic point of view, she was like the aunt of the sons.

And soon we realized that Karen Kigan was hosting two cell lines in her body, cell lines that originated from her own fertilized eggs, that she had, most of her cells were composing of this genetic signature. But some of her cells were originated from a twin sister. So she developed with a twin embryo, but this embryo disappear very quickly, so that's what we call the vanishing twin. But before this embryo disappear, the exchange cells between the twins through the placenta, and the cells from her twin sister eventually handed in her ova. So some of her oversights were carrying her own genetic signature,

and some of the oversights were carrying her sister genetic signatures. So that's how she managed to give birth to her nephews, and nephews whose genetic mother never lived. I couldn't really believe at the first time when I saw those cases, because we know around 10 cases in the world where microchimeric cells have settled in gametes, blurring the genetic filiation. And so this could even invalidate paternity or maternity test. So that's really intriguing.

Yeah, so I'm sort of interested in this because I actually have a twin brother. Okay. But we're, you know, we're non-identical. Yeah, exactly. If you were identical twin, then you couldn't tell that you have cells from your identical twins because that's exactly the same cells. Yeah, do we know how common this is? Do I have cells from my brother and he has cells from me? Yeah, it looks like pretty sure you have cells from your brother and he has cell from you.

And actually, that's how we discover this phenomenon. There were a woman in UK who gave her blood, and we discovered in her blood some blood cell type that were not the same group than she was. and we realized that these blood cells were coming from her twin brother, and her brother died when they were really young. So it was a really tough story for the woman. But that's how we discover that we can exchange cells between twins. So sort of moving through generations, it can also go from grandmother to mother to grandchild.

What can you tell us about that? So that's a discovery from a French team in 2021. They discovered grandmother's cells in the blood of newborn. Well, actually, it was in the cord blood, in the umbilical cord blood. The thing we have to understand is that all the cells within the mother organism can travel through the placenta and settle in the embryo. And within the mother organism, you find her own cells, but you will find also her mother cells because she was also in a womb when she was an embryo. So you can acquire a cell from your grandmother that have settled in your mother organism.

But with this mechanism, you can also acquire cells from your elders because every pregnancy will result in cell exchange. So your elders, either lived elders, but also miscarriage or abortion, will result in cell exchange. And that's how you can acquire cells from all the embryo that have preceded you in the same womb. Peak pollination season and my business is scaling fast. To keep the nectar flowing, I need a phone plan with top priority data speeds. That's why I chose GoogleFi wireless. My connections stay strong even when the hive is buzzing. Plus, unlimited plans started $35 a month.

Now that's a deal that doesn't stay. Explore GoogleFi Wireless plans today. Plus taxes and government fees. Google Fi Wireless is not subject to data traffic deprioritization during times of high network usage. You said this place was steps from the water. We just haven't found the steps yet. How much did we save?

Enough. Enough to get lost! or you could book a stay with Hilton. Welcome to your oceanfront room. Just steps from the water. The Hilton sale is on now. Book on Hilton.com or the Hilton app and save up to 20% to get the stay you expected. When you want savings, not surprises. It matters where you stay.

Hilton for the stay. This podcast is sponsored by Name, Audio and Focal. With over 100 years of combined expertise, Name and Focal have been bringing music to listeners just as the artist intended. Since day one, this mantra has shaped every innovation in hi-fi design, technology and acoustic engineering, balancing craftsmanship and tradition with pioneering thinking. Name audio pushes cutting-edge technology to ensure digital precision whilst sustaining Pratt, pace, rhythm and timing, the elusive quality that makes music feel alive and gives it emotional texture. Today, in part of the music,

with French acoustic specialist focal, name audio creates systems that deliver exceptional sound and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit focal powered by name.com for more information. We've been sticking with sort of family bonds there, but also you talk about somebody who had a bone marrow transplant and this same, essentially same process happened there.

So what do we know about that? Yeah. So in my book, I'm telling the story of Chris Long, who is an American guy and he had leukemia in 2015. And he received a bone marrow transplant from a compatible German donor. And because he was working in a scientific police unit, he did some experiment, interesting experiment. He did some genetic test from different. kind of samples, either the blood, the saliva, hair, salmon. And after it's bone marrow transplantation, he discovered that not only his blood cells were coming from his donor,

which was expected. I mean, we are doing this kind of transplantation to replace, you know, cells from the blood. But not only in the blood, he discovers also his donor cells within all the other sampling, so saliva, hair, and even semen. And it's exactly the same mechanism than a pregnancy. For instance, if you get kidney transplantation, the cell from the donor will escape the graft and travel within the body of the host, but cells from the host will also go within the graft. There is a double-way exchange cells here. So can this sort of thing cause any complications for things like crime and forensic science? Yeah, I'm telling this story also in my book where you have, it was after a rape,

the police scientist came and analyzed a summon sample, and they were happy because they found a genetic signature that they already knew. but the problem is that the man responding to this genetic signature were already in jail at that time so it could not be him that raped

the girl and finally they realized that this guy was also a donor for bone marrow marrow transplant towards his brother and the brother was the one who raped the girl

so yes it can sometimes also blur the genetic testing and maybe some cases that have not been elucidate yet could have some microchimeric issue here. So this is all really interesting sort of in and of itself. But what about ways we can use this science in perhaps future therapies and things like that? Well, for now, I will say scientists are exploring two main way of using these cells for for therapy. The first one will be to repair tissue, finding a way to attract this microchimeric cells to accelerate maybe the regeneration of an organ. And the other way will be in transplantation. Maybe we could use this microchimeric cells to improve the tolerance of the graft. That's also

something that some scientists are hoping for. That's just continuing on with that. That's because oftentimes when somebody receives a transplant, their body sort of rejects it. But they seem to be, I mean, not necessarily welcoming these cells, but they have less of a problem integrating them in within their bodies. Is that right? Yeah, that's also a question that is really in the center of many researchers. Why are we not rejecting these cells? Because in a classic view of the immune system, These cells are different from our cells, so the immune cells should be recognized them as foreign cells and attack them. And it's not what happened.

We all carry these cells and we are not in a continuous inflammatory state. So we tolerate these cells. So this shows us several things. First, we understand that if you acquire these cells when you are a fetus, because your immune system is still very immoral. nature, you will consider these microchimeric cells as yourself. So this is quite easy to understand. But on the opposite, if you acquire this microchimeric cells as an adult, for instance, the pregnant women, then your immune system is a major one and shouldn't accept these cells.

What we understand is that the very first cells from the fetus that go to the mother organism, the trophoblastic cells that we were talking at the beginning. They carry some specific proteins on their surface that make them kind of invisible for the immune cells. And this also explains why we are allowing the fetus during nine months without rejecting him. So we have some understanding of why we accept these cells in our body, but there are still some mysteries

because not only trophoblastic cells, making that traveling. But another aspect of the story is that microchimerism, along with the microbiota, show us that we don't have the good vision of our immune system. Usually we see our immune system as an army, you know, patrolling in our body, tracking every foreign element to kill them, to put them away. But that's not how we are functioning. Our immune system is since the beginning, working with some foreign

elements and without this foreign elements, it could not work. It's not cells that are focusing on the fact that elements are coming from outside. But they might focus a lot more about what those cells are doing within us.

If they are creating some problems, then they can react. But if those cells are coming from outside but still cooperating and still discussing with our own self, then it's fine for our immune system. I think it's really important to change our immune system vision because probably the way we are seeing our biological function might influence the way we are behaving in the world. and I do think it's maybe a better way to see us with an immune system which is very tolerant and which is made by the others since the beginning and that need the others than the opposite.

Yeah, just by way of closing it, in the book you have a really nice phrase that I wrote down. And it's the non-self is within us from our very beginnings and for always. Yeah, exactly. We wield ourselves with the authors from our very beginning. And I think it's really important to understand that both the microbial cells and the microchimeric cells are very important for us and that we are made from and with them. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus. That was Liz Bonnude. To discover more about the topics we've just discussed, Check out her book, Hidden Guests, Migrating Cells and How the New Science of Microchimerism

is redefining human identity. If you liked what you just heard, then please do consider subscribing to Insigenius on your preferred podcast platform. If you'd like to see our guests and hosts in person, then please also check out our YouTube channel, at Science Focus. The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines or download us on your app store of choice. You can also find us on Apple News or online at sciencefocus.com. This podcast is sponsored by Name, Audio and Focal. The texture and emotional depth of music can be lost through digital sources or poor signal.

Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist Focal, Name creates high-end audio systems combining innovation with craftsmanship, so you can listen to music, just as the artist's special. intended. Discover more at name audio.com. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank. Enjoy more ways to save at Ralph's, like low prices in every aisle. And when you download

the Ralph's app, you can clip and save more with digital coupons every week. Plus, you can earn fuel points to save up to $1 per gallon at the pump. At Ralph's, you can enjoy more ways to save and more rewards every time you shop. So it's always easy to save big every day with savings and rewards. Ralph's SoCal for over 150 years. Savings may vary by state. Fuel restrictions apply C-Sight for details.