SUMMARYTE‐19, a mouse monoclonal antibody (mAb) against thymic Hassall's body epithelium, was investigated because of its cross‐reactivity with human erythrocytes. Antibody TE‐19 was found to react only with group A erythrocytes, though it reacted with Hassall's body epithelium from donors of all blood group phenotypes. TE‐19 antibody bound preferentially to cells of subgroup A1and reacted only weakly with A2and Ainterythrocytes. Although it apparently bound to A antigen with both glycolipid and glycoprotein backbones, TE‐19 antibody only reacted with A antigenic moieties containing long or branched chain structures. Analysis of TE‐19 reactivity with erythrocyte membrane components using electroblotting and immunostaining techniques showed antibody reactivity with components migrating in the areas of band 4.5 and just ahead of the dye front, consistent with membrane glycolipid. It is possible that all human thymic Hassall's bodies contain epithelial cells which, irrespective of the donor's erythrocyte blood group, bear a carbohydrate antigen similar t
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