Humans are chimeras, carrying populations of cells from their biological relatives within their bodies, according to a new book, "Hidden Guests: Migrating Cells and How the New Science of Microchimerism is Redefining Human Identity," by journalist Lise Barnéoud. These foreign cells, known as microchimeric cells, originate primarily from mothers and children, passed between them during pregnancy via the placenta.

The book details how mothers retain cells from their children, and children retain cells from their mothers, and potentially even from maternal grandmothers, older siblings, or twins. These cells have been discovered in nearly every organ examined, though they are far less numerous than the microorganisms residing in the gut, skin, and other organs. Barnéoud notes that microchimeric cells occur at a ratio of approximately one cell for every 10,000 to 1 million of an individual's own cells.

This phenomenon, termed microchimerism, raises profound questions about individual identity and the interconnectedness of human beings. The presence of foreign cells challenges the traditional understanding of a person as a genetically distinct entity.

The implications of microchimerism are far-reaching, potentially influencing immune responses, disease susceptibility, and even organ function. While the exact role of these cells is still under investigation, research suggests they may play a role in tissue repair and immune modulation. However, they have also been implicated in autoimmune diseases, where the body attacks its own tissues, potentially mistaking the foreign cells for a threat.

The study of microchimerism is a relatively new field, and researchers are still working to understand the full extent of its impact on human health and biology. Future research will likely focus on identifying the specific functions of microchimeric cells in different organs and tissues, as well as exploring their potential therapeutic applications.