MALT lymphoma IgM antibodies characterized as high‐affinity monoreactive RFs. In enzyme‐linked immunosorbent assays (ELISAs), IgM derived from MALT lymphoma salivary gland samples M86, M91, M93‐I, M94, and M96 show specific binding to coated human IgG (A), with different binding affinities (B). Values on the y‐axis are the absorbance measured at 450 nm. Sample M8 is an IgM from a patient with Sjӧgren’s syndrome–associated MALT lymphoma who was analyzed previously and does not show RF activity. Sample CLL57 is an IGV‐gene–unmutated IgM derived from a patient with B cell chronic lymphocytic leukemia (CLL) and shows strong polyreactivity. Black boxes indicate positive ELISA results at all IgM concentrations tested; the gray‐shaded box indicates positive ELISA results at the 2 highest IgM concentrations tested. The IgMs were tested in at least 4 concentrations, i.e., 125, 250, 500, and 1,000 ng/ml. LPS = lipopolysaccharide (see Figure 1 for other definitions).