Radioimmunoassay studies of the cross-reacting antibody of human group O sera.

Cross-reacting antibody was obtained from three recently stimulated group O donors. The biological reactions of cross-reacting antibody were investigated by a study of the binding characteristics of 125I-labelled purified IgG fractions with red cells and by the reactions of whole antiserum or eluates with 125I-labelled blood group glycoproteins. The bulk of cross-reacting antibody activity is restricted to the IgG fraction. Experiments with 125I-labelled IgG and with the radioimmunoassay system revealed that cross-reacting antibody has a higher binding constant than specific anti-A or anti-B. In the radioimmunoassay system complete inhibition of cross-reaction antibody was obtained either with D-galactose or with N-acetyl-D-galactosamine, but the same concentrations of monosaccharides gave only partial inhibition of specific anti-A and anti-B antibodies. The results indicate that cross-reacting antibody binds to a smaller determinant than specific antibody. Investigations with other monosaccharides did not, however, reveal a common inhibitory structure. The ability of cross-reacting antibody to bind to both group A and B antigens may, therefore, be a property of a 'polyfunctional' antibody binding site.