Effect of IVIG on neonatal platelet counts and maternal IgG levels. (A) Platelet counts in pups from breeding cages of WT × WT and β₃^-/- × β₃^-/- mice as a normal control, immunized female β₃^-/- × WT mice (first delivery), immunized female β₃^-/- × WT mice (treated with IVIG or albumin in their first delivery), and immunized female β₃^-/- × WT mice (treated with IVIG or albumin in their second delivery). n = 12-22. (B) IVIG decreased anti–β₃ integrin IgG in the maternal circulation. Sera of female β₃^-/- mice were incubated with 10⁶ WT platelets at a final dilution of 1:100 for 1 hour. IgG anti–mouse β₃ integrin was detected by a flow cytometric assay (n = 2-3). There was no significant difference in antibody level between the first delivery (untreated) and the second delivery (albumin-treated), and between the first delivery and second delivery after IVIG treatment. (C) IVIG was detected by enzyme-linked immunosorbent assay (ELISA) in the sera of pups delivered from the IVIG-treated mothers during their second pregnancy. Sera of pups from the first delivery (ie, before IVIG treatment) were used as a negative control. n = 3-5 mice. (D) Anti-idiotype activity of IVIG was not found. Preincubated sera from immunized pregnant β₃^-/- mice with IVIG (dashed line; MFI = 145.2 in 1:100 dilution) did not decrease antibody-platelet binding activity compared with sera alone (bold line; MFI = 152.0 in a 1:100 dilution). The thin solid line indicates IVIG alone incubated with mouse platelets. The filled area indicates that anti–human IgG-FITC did not bind to WT platelets incubated with IVIG plus antisera. Data are represented as means ± SEM.