Two CEA cross-reacting antigens: differences in absorbing the same anti-CEA serum.

A factor that cross reacts with the carcinoembryonic antigen (CEA), which we call P factor, was isolated from normal human plasma. To demonstrate the difference between this P factor and the nonspecific cross-reacting antigen (NCA), the same anti-CEA serum was absorbed in an identical manner with both the antigens. Absorption was checked by immunohistochemistry by the beta-galactosidase procedure on sections of colonic adenocarcinoma and normal colonic mucosa. The unabsorbed antiserum recognizes both tissues; after absorption with NCA the staining becomes paler on both tissues, but maintains color on the normal colonic mucosa and granulocytes. Only absorption with the P factor will give an unstained field of normal colonic mucosa, thus revealing the tumor structure. Data obtained by us suggest that the NCA (tissue extract) is an antigen that is not suitable to the absorption of anti-CEA serum for immunocytochemistry techniques, whereas the P factor (plasma extract) appears to be utilizable with good results.