BACKGROUND:  Natural blood cell chimerism rarely occurs in humans. The case of a patient who developed transfusion reaction due to the transfusion of chimeric RBCs is reported.

CASE REPORT:  A 61-year-old male patient with blood group O received two units of packed and O-grouped RBCs after elective kidney surgery. Immediately after blood transfusion, the patient developed a hemolytic transfusion attack. The serologic re-examination revealed only a mixed-field pattern of agglutination of RBCs in one of the two transfused units. The donor of this unit was an apparently healthy 24-year-old male with a twin sister. Both of them showed an identical mixture of roughly 95 percent group O and 5 percent group B RBCs by gel agglutination technology and flow cytometry. The results were also confirmed by ABO blood group genotyping.

CONCLUSIONS:  This is the first report of a hemolytic transfusion reaction related to the transfusion of chimeric RBCs.