Every human body contains a small population of cells that are not genetically its own -- [1]cells that crossed the placenta during pregnancy and that persist for decades after birth. These "microchimeric" cells, named after the lion-goat-serpent hybrid of Greek mythology, have been found in every organ studied so far, though they are exceedingly rare: one such cell exists for every 10,000 to 1 million of a person's own cells.

The cells were first noticed in the late 1800s when pathologist Georg Schmorl described placenta-like "giant cells" in the lungs of people who had died from eclampsia. In 1969, researchers detected Y-chromosome-containing white blood cells in people who would later give birth to boys. For more than two decades, scientists presumed these cells were temporary. That changed in 1993 when geneticist Diana Bianchi found Y-chromosome cells in women who had given birth to sons up to 27 years earlier.

The cells appear to have regenerative properties, transforming into blood vessels or skin cells to promote wound healing. They also challenge a central assumption of immunology -- that the immune system classifies cells as either "self" or "non-self" and rejects foreign material. Microchimeric cells should trigger rejection but do not. Higher-than-typical concentrations have been found in people with autoimmune conditions including diabetes, lupus, and scleroderma.



[1] https://www.nature.com/articles/d41586-025-04102-4