RF inhibits rituximab effects by binding the Fc portion of RTX. A. Monoclonal IgM RF has an inhibitory effect on RTX-CDC when RTX (1 μg/ml) has an IgG1 heavy chain, but not when RTX has an IgG3 heavy chain (n = 3). B. Addition of RTX 10 μg/ml to PBMC in serum free media after addition of monoclonal IgM RF (n = 6) or IgA RF (n = 3) inhibits loss of CD19 by RTX-mediated trogocytosis. Inhibitory effects of RF found to be statistically significant (P < 0.05) are indicated by asterisks. Error bars represent mean standard error.