Two cases of autoantibodies that demonstrate mimicking specificity in the Duffy blood group system

Two transfused Caucasian patients presented with possible delayed transfusion reactions. Both patients demonstrated an anti-Fy^a (Fy1) plus anti-Fy^b (Fy2) pattern of reactivity in their sera. The patients’ red blood cells (RBCs) were Fy:1,–2,3. Both had positive direct antiglobulin tests (DAT) with anti-IgG and -C3d. The serum antibodies reacted with the patients’ RBCs drawn when the DATs were negative Both patients’ serum samples showed reactivity with Fy:1,–2 (1 +), Fy:–1,2 (3+), Fy:1,2 (3+) RBCs at the antiglobulin phase (ACT) but were nonreactive with Fy: –1, –2 RBCs. The anti-bodies in both cases were nonreactive with papain- or ficin-treated RBCs -and reacted with Rh_null RBCs, which ruled out anti-Fy3 and anti-Fy5, respectively. These cases suggest mimicking autoantibody specificities in the Duffy system. One case may indicate that the degree of reactivity is dependent on the Duffy phenotype of the RBCs tested (Fy: –1,2 > Fy:1,2 > Fy: –1, – 2). An unusual characteristic seen in both of these cases was hemolysis in the EDTA cell samples but none in the clotted samples.