The placenta is only a relative barrier as it is known that bi-directional cell traffic occurs during pregnancy. Recent studies indicate that cells can persist in the maternal circulation for years after pregnancy. Maternal cells can also persist in her progeny. The presence of cells from another individual is referred to as chimerism and low levels of non-host cells is referred to as microchimerism. Chronic graft-versus-host disease, a syndrome that occurs from chimerism after stem cell transplantation, resembles spontaneously occurring autoimmune diseases including systemic sclerosis, Sjögren's syndrome, primary biliary cirrhosis and sometimes myositis and systemic lupus. A key determinant of chronic, graft-versus-host disease is the HLA relationship of host and donor cells. These observations, when considered together, led to the hypothesis that microchimerism and HLA relationships are involved in the pathogenesis of some autoimmune diseases. Results of initial studies led support to the concept that non-host cells participate, in the pathogenesis of some autoimmune diseases.