Microchimerism has been defined by the presence of a low number of circulating cells transferred from one individual to another. This transfer takes place naturally during pregnancy, between mother and fetus and/or between fetuses in multi-gestational pregnancies. Furthermore, the establishment of microchimerism can also occur during blood transfusion and organ transplants. Microchimeric cells have been implicated in health and disease. Microchimerism has been correlated with the hyporesponsiveness of the maternal immune system towards the fetal allograft and with the longevity of organ transplants. However, maternal microchimeric cells have been implicated in diseases of the neonate including neonatal graft-versus-host disease, severe combined immunodeficiency and erythema toxicum neonatorum. And more recently, microchimeric cells have been implicated in the pathogenesis of autoimmune diseases including systemic sclerosis and myositis.