Mannan binding protein in sera positive for rheumatoid factor

Low levels of mannan binding protein (MBP) may protect against rheumatoid arthritis and other inflammatory conditions associated with agalactosyl IgG. MBP concentrations were measured in 99 sera found to be positive for rheumatoid factor, at least 41 of which were from patients with rheumatoid arthritis, and the values compared to the data from 352 controls. MBP was undetectable in similar proportions of patients' and control sera (4.0 and 4.3%, respectively), and low values generally were similarly distributed in the two groups. Overall, however, the patient values were distorted upwards relative to controls, consistent with an acute-phase response. These findings do not refute the proposed role for MBP-agalactosyl IgG-mediated complement activation in the pathogenesis of rheumatoid arthritis, but certainly provide no support for that hypothesis.