In humans (and perhaps in all Placentals) the most common form is fetomaternal microchimerism (or fetal chimerism) whereby immune cells (mostly T and B lymphocytes, monocytes, macrophages and NK cells) from a fetus pass through the placenta and establish cell lineages within the mother. Fetal cells have been documented to persist and multiply in the mother for several decades After giving birth, about 50-75 % of women carry fetal immune cell lines. Maternal immune cells are also found in the offspring yielding in maternal→fetal microchimerism, though this phenomenon is about half as frequent as the former .