Inside the Science of Microchimerism: When Foreign Cells Become ‘Hidden Guests’

A century ago, German pathologist Georg Schmorl made a startling discovery: cells from fetuses could migrate into their mothers during pregnancy. This phenomenon, now known as microchimerism, involves genetic material from one individual taking up residence in a genetically distinct host. While initially a curiosity, advances in technology over recent decades have revealed that microchimerism is widespread, with implications ranging from twins absorbing into the womb to recipients of bone marrow transplants carrying donor DNA in their blood and semen.

Lise Barnéoud, a journalist and author, explores these findings in her 2025 book, “Hidden Guests: Migrating Cells and How the New Science of Microchimerism Is Redefining Human Identity.” The work has been shortlisted for the 2026 Royal Society Trivedi Science Book Prize. In the book, Barnéoud examines how the scientific community gradually came to terms with the existence of these “minuscule interlopers” and what they mean for immunology, genetics, and evolution.

Barnéoud first encountered microchimerism while researching a previous book on vaccinations. She sought to correct the common misconception that the immune system acts solely as an intolerant army expelling foreign elements. “We are still learning — [we used to have this idea] that our immune system is kind of an army patrolling in our body and expelling any foreign elements,” Barnéoud explained. She identified pregnancy, the presence of microbiota, and microchimerism as key counterexamples demonstrating that the body often tolerates and even requires foreign cells.

According to Barnéoud, the field faced significant hurdles before gaining traction. The primary obstacle was technological; researchers lacked the tools to study tissues at the cellular level until recently. Additionally, the concept challenged traditional views of immunity. “If you see the immune system as a purely defensive and intolerant army, then you can’t think of these foreign cells staying forever in your body,” she noted. There were also social barriers, with early female pioneers in the field sometimes dismissing by male colleagues as focusing on irrelevant “woman’s stuff.”