Rheumatoid factor blocks C1q and C3b deposition. A. Enriched normal human B cells were treated with RTX at varying concentrations in the presence or absence of IgM RF 10 μg/ml for 30 minutes, and then stained for C3b (n = 5). B. Daudi cells were treated with RTX 10 μg/ml for 30 minutes in the presence of healthy serum or RF+ serum, with or without eculizumab 10 μg/ml, followed by staining for C1q (n = 3). Mean fluorescence intensity (MFI) was determined by flow cytometry. Error bars represent mean standard error.