[Show abstract][Hide abstract]ABSTRACT: During pregnancy, cells move between mother and fetus. Because these cells are able to persist in the host organism for decades, it has been proposed that they are capable of causing several autoimmune diseases. This assumption is supported by the fact that multiple autoimmune diseases have clinical similarities to graft-versus-host disease, which in fact is caused by cells that have been transfused iatrogenically. To illuminate this theory, several studies have examined whether patients with certain autoimmune diseases carry fetal and/or maternal cells to a higher extent than the average population. The conditions have been examined most successfully in patients with scleroderma, in which it has been shown that female patients have a higher concentration of fetal cells in their blood than do matched controls. As it has also been shown that these patients have a higher degree of HLA compatibility with their mothers or children for class II loci, it seems possible that transferred cells can circulate unrecognized and after several years become activated and react against the cells and tissues of the host.