Detection of low molecular weight IgM by immunoblot analysis in rheumatoid arthritis.

The immunoblot technique was used to detect low molecular weight IgM (LMWIgM) in the serum and synovial fluid (SF) of patients with rheumatoid arthritis (RA). LMWIgM was detected in 64% of 58 RA sera and in 47% of 17 RA SF. The levels of IgM and rheumatoid factor (RF) were significantly higher in the positive sera of LMWIgM. Sequential studies revealed that LMWIgM appeared in the serum while the titer of RF was high. Our analysis also suggested the presence of several other oligomeric LMWIgM with monomeric IgM.