Response to IVIG treatment is strain background and dosage dependent and FcγRIIB independent. (A) Two- to 4-fold more IVIG is required for IVIG amelioration of ITP in B6 mice (n = 5 per group) compared with Balb/c mice (n = 6 in each group). (B) IVIG (2.5 g/kg) ameliorates ITP in wild-type B6 mice (n = 10). (C) IVIG (2.5 g/kg) ameliorates ITP using non-fully congenic JAX B6 KO mice (n = 9). (D) IVIG (2.5 g/kg) ameliorates ITP using 129S4 mice (n = 4). (E) IVIG (2.5 g/kg) ameliorates ITP using recommended control F2 wild-type mice (n = 7). Results represent the mean ± SEM. (F) IVIG (2.5 g/kg) ameliorates ITP using fully congenic TAC B6 KO mice. Three independent experiments are shown and indicate a 2.2-, 9.75-, and 6.6-fold, respectively, increase in the platelet count at day 2 after IVIG treatment when the platelet count was at nadir. Data are the mean ± SEM for n = 3, 4, and 5, respectively.