Another member, Gryffen, did a lot of reading on Chimeras and put up a thread-load of information on the subject:
Genetic Chimeras
A small number of people have two different ABO blood types. They are not simply AB codominant. Apparently, most of these blood chimera individuals shared a blood supply with their non-identical twin before birth. In some cases, people are unaware that they had a twin because he or she died early in gestation and was spontaneously aborted. As many as 8% of non-identical twins may have chimeric blood. Some people are microchimeric--they have a small amount of blood of a different type in their system that has persisted from a blood transfusion or passed across the placental barrier from their mother before birth. Likewise, fetal blood can pass into a mother's system. This fact has led some researchers to suggest that the significantly higher frequency of autoimmune disorders in women is a result of the presence of foreign white blood cells that had come from their unborn children during pregnancy.
