Every human body contains a small population of cells that are not genetically its own — 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…
