Hidden Guests
Translated by Lise Barnoud and Bronwyn Haslam, Greystone Books

Although my children were conceived using donated eggs, one might presume we lack any genetic connections. Yet, science confirms that this assumption is not entirely accurate.

Research reveals that during pregnancy, fetal cells traverse the placenta and integrate into the mother’s body, affecting various organs. Likewise, my mother’s cells—and even those transferred from her to me—can also reach my children. With an older sister, the cellular sharing could potentially be even more extensive. Her cells may have crossed into my mother, then to me, and subsequently transferred to my children.

This intriguing concept and its ramifications—that we are holobionts, consisting not just of human cells and microbes but also fragments of other cells—lie at the core of the book. Hidden Guests: How Cell Migration and the New Science of Microchimerism Are Redefining Human Identity by Liz Barneau.

Barneau not only documents the serendipitous discovery of these microchimeric cells but also discusses how cultural and political factors have shaped their interpretation. The notion that pieces of a parent, sibling, or even a fetus reside within our bodies or minds can elicit a spectrum of emotions.

“Some find solace in feeling connected to deceased loved ones, while others worry about renewed control exerted by men,” she notes, referencing claims by some anti-abortion advocates that fetal cells “haunt” women post-abortion, causing illnesses and punishing them.

Fortunately, Barneau primarily focuses on scientific evidence. She chronicles the journeys of researchers through their errors, inquiries, and breakthroughs, illustrating how cells derived from fetuses can have both beneficial and detrimental effects. While these cells might facilitate tissue repair and combat tumors, they are also implicated in autoimmune disorders. Barneau handles the more sensitive topics with care, describing how the cells of an aborted fetus can persist in a woman’s body for decades.

Additionally, she explores how their presence can disrupt traditional genetic inheritance patterns, unveiling remarkable biological enigmas. For instance, a woman may use her own eggs to conceive all her sons, yet only a third of her sons may exhibit shared genetic features. Alternatively, a woman with hepatitis C might possess numerous liver cells that correspond to DNA from two former partners, potentially linked to an aborted pregnancy years before. There’s also the case of an Olympic cyclist who tried to explain a suspicious blood type mix by claiming “vanishing twins,” referring to fraternal twins whose DNA merges in utero.

Hidden Guests is clearly articulated and full of insightful analogies. Journalist Barneau likens microchimeric cells in our bodies to stars in distant galaxies that “bear molecular signatures distinct from ours.” She draws parallels stating that blaming a cell for a tumor’s growth is akin to assigning fault for a fire to a firefighter.

Many revelations await readers. Spoiler alert: Barneau encourages contemplation on the fact that, just as donor cells migrate from a transplanted organ to various parts of a recipient’s body, cells containing a partner’s genetic material can enter blood and lymphatic vessels surrounding the vagina, embedding themselves across the body or brain.

This genetic ambiguity, wherein cells journey back through familial lines, can indeed be perplexing. Nevertheless, Barneau excels at elucidating the current landscape of this nascent field and its significant implications for both medicine and human identity without veering into speculation. She adeptly unravels the long-standing perspective of “one individual, one genome” in an engaging manner.

As a mother who once believed I lacked any biological link to my children, I now realize: Hidden Guests is scientifically enthralling and profoundly reassuring. Barneau enlightens us that we carry traces of others within us. Thanks to her insights, I eagerly anticipate the future developments in this field.

Helen Thomson, a writer based in London, columnist for New Scientist