Quantitation of human serum polymeric IgA, IgA1 and IgA2 immunoglobulin by enzyme immunoassay.

This report concerns the relative quantitation of serum polymeric IgA and polymeric IgA subclass concentrations by enzyme immunoassay (EIA). The assay relies on the specific binding of polymeric IgA to secretory component. Competition between pentameric IgM and polymeric IgA for binding to secretory component was observed. Thus, samples were adsorbed for IgM by affinity chromatography before the EIA was performed. The assay was used to determine an age-related range of serum polymeric IgA concentrations and to compare the polymeric IgA concentrations in patients with IgA nephropathy (n = 50) to those of controls (n = 50). The serum concentrations of both polymeric IgA and polymeric IgA1 increased with age reaching adult values of around 12 years of age. Polymeric IgA2 concentrations did not reach adult levels until 18 years of age. The ratio of the polymeric IgA concentration to the total serum IgA concentration was found to be significantly increased in children under 2 years of age compared with those over 4 years of age (Mann-Whitney U-test, P less than 0.01). Patients with IgA nephropathy had significantly increased concentrations of polymeric IgA (P = 0.001) and polymeric IgA1 (P = 0.001) but similar polymeric IgA2 concentrations to controls.