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 of125I‐labelled purified IgG fractions with red cells and by the reactions of whole antiserum or eluates with125I‐labelled blood group glycoproteins.The bulk of cross‐reacting antibody activity is restricted to the IgG fraction. Experiments with125I‐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‐reacting 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 ‘
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