Tumor Marker Antigens for Immunoassays: CEA, AFP, CA125 and CA19-9

Selecting antigen material for CEA, AFP, CA125 and CA19-9 immunoassays: what differs between these tumor markers at the bench.

Four Markers, Four Biochemistries

CEA, AFP, CA125 and CA19-9 are widely used tumor markers, but they are chemically distinct. CEA is a glycoprotein of the immunoglobulin superfamily; AFP is a fetal plasma protein; CA125 (MUC16) is a large mucin; and CA19-9 is a carbohydrate antigen, a sialylated Lewis structure carried on glycolipids and glycoproteins. Treating these analytes as interchangeable protein markers is a common early mistake. Because they differ in nature, no single assay strategy suits all four. Antigen choice and antibody design must follow the biochemistry of each analyte, and the calibrator must present the epitope the antibody recognises.

Antigen Source and Heterogeneity

For protein markers such as CEA and AFP, recombinant or purified native material can be used, and the choice affects which epitopes are present. Glycosylation varies with the expression system, and some antibodies recognise carbohydrate-dependent epitopes while others do not. For a carbohydrate antigen such as CA19-9, the analyte is not a single protein, so calibrator material must present the correct epitope in a defined form. Establish which epitope your antibody recognises before committing to a calibrator.

Standardisation and Calibrators

Tumor marker assays have historically been difficult to harmonise, because different methods measure different forms and report different numerical results for the same sample. Calibrators should be traceable and their composition documented, including the form of the antigen and the matrix. Gold and Freedman described CEA in the Journal of Experimental Medicine (1965); Bast and colleagues reported the CA125 assay in the New England Journal of Medicine (1983); and Magnani and colleagues characterised the CA19-9 epitope in Science (1981).

Ask a technical question