Journalist and author Lise Barnéoud has written a book titled Hidden Guests, which delves into the fascinating concept of microchimerism — a biological phenomenon where cells from one individual can live within another. First observed over a century ago by German pathologist Georg Schmorl, microchimerism challenges traditional understandings of the human body and its boundaries. It has been found that people can carry cells from a grandmother, the genetic remnants of a lost twin, or even a donor’s cells after a bone marrow transplant. Barnéoud’s book has been shortlisted for the 2026 Royal Society Trivedi Science Book Prize, highlighting its significance in the scientific community. In an interview, Barnéoud explained how microchimerism reshapes our understanding of the immune system, which is not as rigidly protective as once believed. Microchimeric cells — those that have migrated from another person — can multiply, produce proteins, and even influence bodily functions. However, the field has faced challenges, including outdated technology, resistance to new ideas about immunity, and a lack of recognition for early researchers, many of whom were women. These obstacles have slowed progress, but recent advances in genetic testing have opened new avenues of study. Barnéoud shared two compelling real-world examples of microchimerism’s impact. One involved Lydia Fairchild, a woman who nearly lost custody of her children because her DNA did not match that of her children — a result of microchimerism, where cells from her fetus had remained in her body. Another case in Alaska helped identify a rapist through the presence of a bone marrow transplant recipient’s cells in semen, demonstrating the phenomenon’s potential in forensic science. These stories show how microchimerism can complicate or aid in DNA-based conclusions. The concept has also sparked debate in social and political spheres, appearing in discussions around abortion rights and even in the rhetoric of certain online communities. Barnéoud emphasized that the field is still evolving, with many hypotheses yet to be tested. For example, some researchers speculate that cells from a sexual partner might be passed on to offspring, but this remains unproven. Looking forward, Barnéoud sees promise in using microchimerism for medical applications, such as tissue repair and improving the success of organ transplants by reducing the need for immunosuppressive drugs. The science of microchimerism continues to unfold, offering new perspectives on human biology and identity.