The binding of carcinoembryonic antigen by antibody and its fragments

In order to assess the potency of antigenic fragments of carcinoembryonic antigen (CEA) in the radioimmune assay; it is necessary to know whether the high affinity of goat anti-CEA antibody (which makes possible the detection of as little as 10--11M CEA) is due to bivalent binding of the CEA molecule. Immunoglobulin G and the F(ab')2 and Fab fragments derived from it were prepared from an anti-CEA serum and tested for their abioity to bind CEA. Equivalent concentrations of binding sites of the bivalent F(ab)2 and univalent Fab fragments of anti-CEA were identical to the immunoglobulin G fraction in the standard inhibition curve. Fragments of CEA obtained by trypsin digestion produced equivalent inhigition curves when tested with either immunoglobulin G, F(ab')2, or Fab". This, increased avidity due to bivalent binding to a single antigen molecule cannot be invoked to explain the sensitivity observed in the CEA assay. This high sensitivity implicates the protein rather than the carbohydrate as an important part of the antigenic determinant(s) of CEA.