Hidden Guests: Migrating Cells and How the New Science of Microchimerism is Redefining Human Identity Lise Barnéoud, transl. Bronwyn Haslam Greystone Books (2025)
The chimaera of Greek mythology was “an evil creature with a lion’s head, a goat’s body, and a serpent’s tail”, notes journalist Lise Barnéoud in Hidden Guests. Humans are also chimaeras — thanks to the presence of cells that are not our own inside our bodies.
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Mothers carry cells that came from their biological children, passed across the placenta when the baby was in the womb. Likewise, children carry cells that were transferred to them in utero from their mothers — some of which might even be from the child’s maternal grandmothers, older siblings or twin.
These ‘microchimeric’ cells have been found in every organ that has been studied so far. But they are also rare — much rarer, for example, than the trillions of microorganisms that reside in our guts, on our skin and in many other organs. We carry only one microchimeric cell for every 10,000 to 1 million of our own cells.
In Hidden Guests, the author invites readers to learn about the pioneering “microchimerists” — scientists who discovered these fascinating shared cells. She also challenges us to consider the broader implications for health and science, and the philosophy of the fact that we are all chimaeras.
Unplanned discovery
Microchimeric cells were, we learn, discovered through a series of accidental observations. In the late 1800s, pathologist Georg Schmorl described ‘giant cells’ in the lungs of people who had died from eclampsia — a life-threatening inflammatory condition that can occur during pregnancy. These giant cells resembled the cells of the placenta, leading Schmorl to suggest that fetal cells passing into the bloodstream of mothers was the norm, rather than the exception.
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Then, in 1969, a team studying immunity in pregnant people detected white blood cells that contained the Y chromosome in the blood of individuals who would eventually give birth to boys1. For more than two decades, it was presumed that these microchimeric cells were a temporary feature of pregnancy. It wasn’t until 1993 that geneticist Diana Bianchi found cells with Y chromosomes in women who had given birth to sons between one and 27 years earlier2.
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