The antigenicity of carcinoembryonic antigen in man

Binding of globulins to -125I CEA was detected in 10 of 24 sera from patients with cancer, 13 of 26 sera from blood donors, and 7 of 27 supernates of tumor-cell suspensions. Binding was predominantly by IgM in 10 of 14 sera tested by radioimmunoelectrophoresis. Binding was inhibited by 6 ng unlabelled CEA in one instance only, and by adsorption with blood group A erythrocytes in three cases. Binding was not increased in dialysed precipitates formed by ammonium sulfate at pH 2.5, while there was increased binding in precipitates of complexes of CEA and heterologous anti-CEA. These results confirm that human IgM binds to CEA, and in some cases binding appears to be mediated by antibodies to blood group A. However, the binding is weak and appears to be of low affinity. This suggests that binding of CEA by immunoglobulins represents cross-reactivity by antibodies to similar antigens, of which blood group A is one example.