Individuals who experience traumatic injury often require blood transfusion. In some individuals who receive blood after an injury, white blood cells from a person who donated blood may remain in the body for years, a condition known as microchimerism. This study is designed to examine a group of people who are known to have long-term microchimerism and, through analysis of their blood, determine whether there is evidence that the microchimerism involves blood stem cells that can become any type of blood cell (red blood cells, white blood cells, or platelets) and that might be a permanent part of the body.