 Approximately 20 of individuals who suffer a traumatic injury require a blood transfusion; of these, 10 to 15 experience a condition known as transfusion-associated microchimerism. This occurs when white blood cells, or leukocytes, from the donors blood persist in the recipient long after the transfusion occurs. The genetically distinct donor cells can remain in the individual for decades, and may account for as many as 4 of the white blood cells in the recipients body. This suggests that the donor cells are tolerated by the recipients immune system. The purpose of this study is to examine the specific factors that predispose some blood transfusion recipients to develop microchimerism. 