The absence of FcRn does not result in reduced in vivo antiplatelet antibody binding to platelets or affect the ability of IVIg to inhibit ITP. C57BL/6 wild-type mice and FcRn-deficient mice were injected with antiplatelet antibody alone or antiplatelet antibody 30 minutes after IVIg treatment. Mice were bled at the indicated times for antiplatelet antibody staining or platelet enumeration. Platelets obtained from C57BL/6 mice (A,C) injected with antiplatelet antibody alone (□) or IVIg + antiplatelet antibody (▵) and FcRn-deficient mice (B,D) injected with antiplatelet antibody alone (□) or IVIg + antiplatelet antibody (▵) were stained with a fluorescent anti–rat IgG antibody and analyzed by flow cytometry for mean log channel fluorescence intensity (MLFI; A-B) or percentage positive platelets (C-D) of antiplatelet antibody binding. The x-axis indicates the time of bleeding after antiplatelet antibody injection; and y-axis, the MLFI of antiplatelet antibody binding (A-B) or percentage positive platelets (C-D). n = 8 mice per group from 4 independent experiments. Data are mean ± SEM. Platelet-rich plasma from the C57BL/6 mice (A,C) and FcRn-deficient mice (B,D) were used to enumerate platelets by a Z2 Coulter Counter in panels E and F, respectively.¹⁹  The x-axis indicates the time of bleeding after antiplatelet antibody injection; and y-axis, platelet count. n = 8 mice per group from 4 independent experiments. Data are mean ± SEM.