IgG antibodies do not bind to band 3 in intact erythrocytes; enzymatic treatment of cells is required for IgG binding.

Naturally occurring autoantibodies to band 3 in normal human serum do not bind to red cells unless they are senescent, stored, or damaged. We suspected that IgG did not bind to native band 3 in intact red cells because of steric "shielding" of band 3 by adjacent molecules such as the glycophorins. In order to test this hypothesis, immunoelectron microscopy experiments were performed on intact red cells and red cells subjected to enzymatic treatment. Results revealed that antibodies to band 3 did not bind to untreated red cells, but did bind to red cells treated with trypsin, which spares band 3 but cleaves glycophorins A and C. Treatment with alpha-chymotrypsin did not significantly increase antiband 3 antibody binding. Binding of antibodies to the senescent cell antigen was not increased by either enzymatic treatment. It appears that binding of antibodies to the senescent cell antigen requires more than exposure of band 3 and proteolysis at a site other than the alpha-chymotrypsin sensitive site in intact red cells.