IgM rheumatoid factor does not prevent RTX attachment to CD20. A. RF inhibits B cell death whether RTX is first added to cells followed later by RF addition (Stepwise), or if RTX and RF are added simultaneously (n = 3). Error bars represent mean standard error. B. Representative data (n = 2) showing that RTX blocks binding of anti-human CD20 APC-Cy7, and addition of RF to RTX does not lessen this blocking outcome.