Transfusion-associated microchimerism, the persistence of donor white blood cells months or years after transfusion in the recipient, has been observed in approximately 10-15% of transfused trauma patients. Previous studies suggest that the microchimeric cells include multiple immunophenotypes of leukocytes (CD4+, CD8+, CD15+, and CD19+) and that they can persist for decades, features suggestive of hematopoietic engraftment.