Microchimerism: sharing genes in illness and in health.

Microchimerism is defined as the presence of two genetically distinct cell populations in the same individual. It can arise from several causes including the bidirectional transfer of cells between mother and fetus during pregnancy, twin-to-twin transfer in utero, from organ transplantation, and blood transfusion. Recently, scientists have found male fetal cells from decades earlier imbedded in tissues and organs of some women with autoimmune diseases. The significance of these findings as they relate to real or potential health implications in autoimmune diseases, graft-versus-host reactions, and transfusion complications is discussed here.