Allergenic and antigenic properties of bovine serum albumin

Abstract

The antigenic sites on bovine serum albumin were studied utilizing peptic and tryptic fragments of the molecule. Rabbits were immunized with small doses of bovine serum albumin and their serum antibody response measured by a bincling assay and the IgE response by antigen-induced histamine release from their basophils. The basophils from two bovine serum albumin allergic individuals were also used for histamine release studies. Serum antibodies bound large fragments of bovine serum albumin; these fragments also induced histamine release from basophils. Although about half of the antibody bincling activity was recovered in the two halves of the albumin molecule only about 10% of the histamine releasing activity was present in the same fragments. The loss of activity of the bovine serum albumin molecule on proteolytic cleavage into two halves could be due to the breakup of the molecule in the middle of the third domain with loss of antigenic sites and/or due to minor conformational changes in the fragments as compared to the intact molecule. Large fragments of bovine serum albumin induced basophil histamine release, thus demonstrating the presence of at least two antigenic determinants on each of these peptides. This data therefore suggests the presence of at least four antigenic IgE-bincling sites on the bovine serum albumin molecule. By basophil desensitization experiments unique IgE-reactive antigenic sites were demonstrated on each half of the molecule; however, some of the sites on the COOH-terminal half cross-reacted with antibodies directed towards the NH 2-terminal part of the molecule. The IgE-response of rabbits to bovine serum albumin was specific; there was no cross-reactivity with rat or mouse albumin. The present finclings indicate a substantial loss in the IgE-reactive determinants of bovine serum albumin by cleavage into large fragments.