[Determining the level of the rheumatoid factor associated with polyclonal and monoclonal human serum IgA].

The authors studied the activity of the rheumatoid factor (RF) associated with polyclonal and monoclonal IgA contained in the sera of patients with chronic diseases of noninfectious etiology and A-paraproteinemia. IgA-RF-activity was determined by an enzyme immunoassay. In most of the patients' sera IgA had a high level of RF-activity. The frequency of RF-activity detection did not depend on the level of polyclonal IgA but correlated with the level of monoclonal serum IgA. During a study of the chromatographic fractions of sera containing monoclonal IgA, RF-activity was mainly revealed in the fraction of IgA polymers; IgA monomers forming the bulk of serum IgA, demonstrated no RF-activity. The inclination of curves reflecting the correlation between the level of IgA-RF and dilutions of tested sera made it possible to assess function of the affinity of such molecules which was growing with an increase in IgA molecular mass.