Protein A reactivity of two distinct groups of human monoclonal IgM

Fifteen human monoclonal IgM proteins could be divided into two groups based on their ability to inhibit binding of a labelled human monoclonal IgM (IgM-Se) to Staphylococcus aureus Cowan 1 (IgM-Se assay); seven were positive and eight were negative. We found complete agreement between positive reaction in the IgM-Se assay, the presence of protein A reactivity in gel diffusion experiments, and high titres in an indirect haemagglutination test detects antibodies to protein A-IgG complexes. The protein A precipitation patterns of IgM and autologous IgG were not identical.