Laser nephelometric estimation of
IgM in the eluate fractions following
Sepharose 6B chromatography has enabled the calculation of the proportion of low molecular weight
IgM (7S
IgM) in normal and pathological sera. This figure has then been used to determine the absolute amount of 7S
IgM. Twenty-seven of 36 (75%) patients with
rheumatoid arthritis had 7S
IgM with a mean value of 17 mg/100 ml (170 mg/l) (range 2.5-59 mg/100 ml). No sera from 10 healthy controls were found to contain 7S
IgM. Patients with active
rheumatoid arthritis had significantly more 7S
IgM than those with inactive disease, but there was no significant difference between those patients with and without
rheumatoid vasculitis. Significant correlations occurred between 7S
IgM and the absolute
IgM level (P < 0.01), the Rose-Waaler titre (P < 0.01), and the erythrocyte sedimentation rate (P = 0.01). However, there was no significant correlation with the age of the patient, the duration of the disease, or the level of circulating
immune complexes as measured by the Clq binding assay. It is concluded that 7S
IgM commonly occurs in
rheumatoid arthritis, and it is postulated that a common immunological stimulus is responsible for the production of 7S
IgM and rheumatoid factors, serological abnormalities that characterise this disease.