IVIg ameliorated ITP associated with ameliorated platelet-leukocyte engagement. IVIg (1 g/kg) was pretreated 50 minutes to ameliorate anti-CD41 Ig (rat anti-mouse mAb, MWReg30; 0.1 mg/kg) induced ITP and platelet-CD14⁺ leukocyte (PLT-CD14⁺ LC) engagement of C57BL/6J mice. The experiment outline is illustrated in panel A. PLT counts of mice with different genetic backgrounds, WT (B), Fcgr2b^−/− (C) and Fcgr3^−/− (D), at various time intervals after ITP inductions are indicated. *P < .05 versus vehicle groups. n = 8 (4 experiments with 2 replicates). In vitro engagements of lineage specific (PLT: CD41; LC: CD14) fluorescent Igs labeled PLTs and LCs from respective mice groups (B-D) were analyzed using flow cytometry (E-F). LCs were first distinguished from PLTs using cell size and granularity characteristics (Ei, R1 region), and then PLT-engaged LCs were quantified by measuring CD14⁺ CD41⁺ double positive cells (top right regions in panel Eii-iii). Quantified results of PLT-CD14⁺ LC engagements are indicated (% of double positive cells) (F); *P < .05 versus respective vehicle groups. n = 8 (4 experiments with 2 replicates). Data are mean ± SD.