A subject of group O was found to produce A-B cross-reacting antibody in response to the injection of a preparation of blood group B substance free of blood group A activity. Both anti-A and anti-B could be precipitated from the serum with either blood group A or B substance, and precipitation of either A or B substance could be inhibited by dialyzable oligosaccharides from either antigen. Evidence is presented to indicate that the specificity of the cross-reacting antibody is directed against structural features common to the oligosaccharides which determine blood group A and B specificity. The possible antigenic sites involved in the cross-reaction are discussed.

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