Recent results from J Lee Nelson and colleagues (Sci Am. 2008 Feb;298(2):64-71; available as full text through Google) show that cells from the mother were found in a person aged 46. Logically, these cells should be stem cells, as any other cells would die in such a long period of time. So, what stem cells could be transferred from mother to fetus? Why doesn't the immune system reject those cells?
Since the maternal cells can be found in a number of tissues including
In second trimester fetuses the thymus, lung, heart, pancreas, liver, spleen, kidney, adrenal gland, ovary, testis and brain had maternal Mc (microchimerism)
it's clear that these are stem or stem like cells that are being incorporated into fetal tissue.
Since these cells arrive while the immune system is developing, they are considered as self and not foreign. But a breakdown of that immune tolerance is thought to be involved in a number of autoimmune diseases, especially systemic sclerosis which resembles a host vs graft disease where the patient's immune system attacks tissues holding the maternal cells.