The specific detection of IgA in immune complexes.

An assay to detect IgA in circulating immune complexes (IC) using low avidity goat IgM antibody against human polyclonal IgA is described. The binding of this antibody to IgA coupled to Sepharose 6B is inhibited by IgA-containing IC. The specificity and sensitivity of this anti-IgA inhibition assay (a-IgA-InhA), was evaluated with aggregated purified immunoglobulins, sera of patients with Henoch-Schönlein purpura and normal human sera. Aggregated immunoglobulins of the IgA class, but not monomeric IgA, were reactive. Sucrose density ultracentrifugation showed that large IgA constituents (greater than 19S) were found only in the sera of patients with Henoch-Schönlein purpura. Both these sera and normal human serum contained smaller IgA components (between 7S and 19S), probably small polymers of IgA, which were reactive in this assay and interfered with detection of IgA-containing IC. Redissolved precipitates obtained from normal serum with polyethylene glycol showed reduced reactivity in the test, whereas the inhibitory activity of IgA-containing IC in sera of patients with Henoch-Schönlein purpura was retained in the precipitates. Precipitation of sera with polyethylene glycol allowed detection of smaller quantities of IgA-containing IC in patients with Henoch-Schönlein purpura.