Modulation of the KEL antigen on transfused RBCs in the presence of immunoprophylaxis requires FcγRs or C3. KEL RBCs labeled with the lipophilic dye DiO were recovered from recipients transfused in the absence or presence of anti-KEL immunoprophylaxis 10 minutes, 1 hour, and 24 hours after transfusion (A) and incubated with anti-KEL sera followed by fluorescently conjugated anti–mouse IgG (schematic shown in B). (C) Detection of the KEL antigen on recovered lipophilically labeled KEL RBCs in wild-type, FcγR KO, C3R KO, and FcγR × C3 KO (double-KO) recipients, transfused in the presence or absence of anti-KEL immunoprophylaxis (P < .05 at 1 hour and 24 hours after transfusion by 2-way ANOVA between double-KO and other recipients treated with immunoprophylaxis). These data are representative of 2 or 3 independent experiments with 3 mice per group per experiment; error bars indicate SD between mice.