An enzyme immunoassay for the determination of anti-IgA antibodies using polyclonal human IgA.

An enzyme immunoassay (EIA) for screening and quantitation of serum anti-IgA antibodies of IgG class is described. This method is based on the use of purified polyclonal human serum IgA as the coating antigen and a commercial alkaline phosphatase-conjugated anti-human IgG as the detecting antibody. Nonspecific reactions were minimized by blocking vacant protein binding sites with bovine serum albumin and by using individual sample blanks. The IgA specificity of a positive antibody finding was confirmed by testing inhibition: pooled normal human serum inhibited the binding of specific antibodies by over 80%. The same degree of inhibition could also be demonstrated by a commercial myeloma IgA preparation and by the IgA used for coating but not by IgA-deficient serum (< 0.05 mg/l). On the basis of the mean anti-IgA antibody titre in EIA, a value of 12,000 arbitrary units of anti-IgA per litre (AU/l) was assigned to a patient serum used as standard in the assay. Anti-IgA results obtained by EIA and haemagglutination correlated well, which makes it possible to compare earlier HA results with those obtained now by EIA. The measuring range of the assay was 0.6-27 AU/l and the lowest quantifiable concentration 7 AU/l. The dilution requirement for serum was 1/16. The interassay coefficients of variation for control sera with antibody levels from 35 AU/l to 3770 AU/l varied from 9 to 12%.