Anti-erythrocyte antibodies inhibit the functional expression of the activating FcγRIIIA on splenic macrophages. (A) FcγRIIB^–/– mice were injected with 2 g/kg IVIg intraperitoneally or 50 μg TER119, M1/69, or 30-F1 intravenously. After 24 hours, splenic cells were isolated and incubated with FITC-2.4G2 and PE-CY5-F4/80. The x-axis denotes the treatments given to each group of mice, the y-axis denotes FcγRIIIA expression (mean channel fluorescence) on F4/80-positive cells. Nil indicates no treatment. n = 5 for each column. *P < .005 versus Nil. (B) FcγRIIB^–/– mice were injected with 2 g/kg IVIg intraperitoneally or 50 μg 2.4G2 or control IgG (rat IgG) intravenously. After 2 hours, 50 μg TER119 was injected, and erythrocyte count was evaluated on flow cytometry. The x-axis denotes the treatments given to each group of mice, the y-axis denotes the erythrocyte count. Nil indicates no treatment. n = 7 for each column. In both panels A and B, data are expressed as mean ± SEM.