Affinity in radioimmunoassay of antibody cross-reactive with carcinoembryonic antigen (CEA) and colon carcinoma antigen-III (CCA-III).

Several antisera raised against purified carcinoembryonic antigen (CEA) were evaluated for their content of antibody cross-reactive with colon carcinoma antigen-III (CCA-III). All antisera gave a reaction of partial identity between CEA and CCA-III and demonstrated a high titer in CEA radioimmunoassay (RIA). Between 50 and 70% of the CEA RIA activity was removed, however, by absorption with soluble CCA-III or adsorption onto CCA-III-containing immunoadsorbents. Immunoadsorbent retained antibody gave a line of complete identity between CEA and CCA-III. Purified CCA-III (2 mug) only partially depressed CEA binding by this common site antibody, whereas nanogram quantities of CCA-III inhibited the reaction between specific CCA-III antibody and radioiodinated CCA-III. In addition, low levels of CEA were equally effective in depressing CEA binding by the common site or CEA-specific antibody. The higher affinity in RIA of the common site antibody for CEA over CCA-III suggests that the common determinant expressed on CEA is stereochemically different from that on CCA-III. The results further demonstrate that interference by plasma CCA-III is not a significant factor in the measurement of CEA by RIA.