Bidirectional cell trafficking occurs between a mother and fetus during pregnancy. This trafficking is associated with the persistence of non-self cells and is termed chimerism or, because of the low levels of non-self cells, microchimerism. Persistence of these cells has been demonstrated for many years after birth in the child and mother. Children with juvenile dermatomyositis, juvenile idiopathic inflammatory myositis, and healthy adults have all demonstrated persistence of maternal microchimerism, which is increased in the diseased population and thought associated with human leukocyte antigen genes of the offspring and the mother.