Sera from patients with ulcerative colitis contain antibodies which hemagglutinate sheep red cells, sensitized with phenol-water extracts from. colon, cecum, or feces of germfree rats. Minor concentrations of such antibodies are also present in a certain fraction of normal human sera. Hemagglutination and hemagglutination inhibition experiments with human erythrocytes and with the rat extracts showed that the latter contained an antigen similar to human blood group A antigen. In contrast, a blood group B-like antigen could not be detected in these extracts. However, experiments with eel serum indicated that these extracts also contained an antigen similar to the H antigen of the human ABO system.

Absorption of ulcerative colitis sera with human A1 erythrocytes but not that with B or O erythrocytes gave, in a few cases, a slight reduction of the hemagglutinating titers against rat cecum-sensitized sheep erythrocytes. In contrast, this treatment considerably reduced such titers when found in sera from healthy persons or from patients with unrelated diseases. It could be concluded that the rat extracts also contained a "colon" antigen, detected with antibodies, present at elevated titers, in the sera of ulcerative colitis patients, but not in those of the controls. This colon antigen is immunologically distinct from the blood group antigens studied.

Hemagglutination inhibition experiments indicated that A, H and colon antigen were widely distributed throughout the gastrointestinal tract of the germfree rats. The colon antigen was found to be enriched in the extracts from colon, cecum, and feces. Fluorescent antibody staining provided evidence that both the colon antigen and the A antigen were present in similar sites of the colon and cecum mucosa, particularly in goblet cells of the crypts, and in the mucus.

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