Characterization of Precipitating Antibodies to Ruminant Serum and Milk Proteins in Humans with Selective IgA Deficiency

Abstract
Serums from 23 patients with isolated IgA deficiency contained precipitating antibodies to ruminant IgM proteins. Purified IgM proteins from bovine serum as well as similar proteins in other ruminant (goat, sheep, deer and elk) serum and in cow's milk reacted with the patient serum. IgA-deficient serum did not precipitate serum proteins from humans, nonhuman primates or numerous other animals. Patients with diseases reported to be associated with milk precipitins (e.g., Down's and Heiner's syndromes) may or may not have this factor in addition to antibodies directed toward other bovine milk and serum proteins. Because of the increased prevalence of rheumatoid arthritis in IgA-deficient persons this "reversed rheumatoid factor" (an IgG antibody to IgM proteins) in many IgA-deficient serums is noteworthy. This factor, however, will be missed in IgA-deficient serum if radial immunodiffusion tests with goat anti-IgA serum are used for the quantitation of IgA. The test will be misinterpreted and thought to show low IgA concentrations.

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