Blood-group–specific antibodies present in IVIG products are of the IgG₂subclass and cause opsonization of erythrocytes of the A, B, and AB blood group. (A) Antibody titers against A and B blood group antigens in product II, product III, product IV, product V, and product VI. (B) Erythrocytes of blood group O and blood groups A, B, or AB were opsonized with either of the 4 IVIG products mentioned (10 mg/mL) and subsequently stained with fluorescently labeled goat-anti-human immunoglobulin. The median fluorescence intensity (MFI) is shown (n = 14-23 erythrocyte donors). (C) Erythrocytes of the O and A blood group were opsonized with product IV or product V and stained with subclass- and isotype-specific antibodies: anti-IgG₁, anti-IgG₂, anti-IgG₃, anti-IgG₄, anti-IgG, anti-IgM, and anti-IgA. The MFI is shown. Graphs are representative graphs of 2 independent experiments.