Carcinoembryonic antigen (CEA) is a glycoprotein involved in cell adhesion. It is normally produced during fetal development, but the production of CEA stops before birth. Therefore, it is not usually present in the blood of healthy adults, although levels are raised in heavy smokers. CEA is a glycosyl phosphatidyl inositol (GPI)-cell surface anchored glycoprotein whose specialized sialofucosylated glycoforms serve as functional colon carcinoma L-selectin and E-selectin ligands, which may be critical to the metastatic dissemination of colon carcinoma cells.
It was found that serum from individuals with colorectal carcinoma, gastric carcinoma, pancreatic carcinoma, lung carcinoma and breast carcinoma, as well as individuals with medullary thyroid carcinoma, had higher levels of CEA than healthy individuals (above 2.5 ng/ml).
CEA elevation is known to be affected by multiple factors. It varies inversely with tumor grade (well-differentiated tumors secrete more CEA). CEA is elevated more in tumors with lymph node and distant metastasis than in organ-confined tumors (varies directly with tumor stage). Left-sided tumors tend to have higher CEA levels than right-sided tumors. Tumors causing obstruction produce higher CEA levels. Aneuploid tumors produce more CEA than diploid tumors. Liver dysfunction increases CEA levels as the liver is the primary site of CEA metabolism.
CEA levels are higher for smokers than for nonsmokers. Other nonneoplastic causes of elevated CEA include inflammatory bowel disease, peptic ulcer disease, pancreatitis, biliary disease, liver dysfunction (as stated above) and hypothyroidism. 
CEA measurement is mainly used as a tumor marker to identify recurrences after surgical resection, or localize cancer spread through dosage of biological fluids. The CEA blood test is not reliable for diagnosing cancer or as a screening test for early detection of cancer. Most types of cancer do not produce a high CEA. Elevated CEA levels should return to normal after successful surgical resection, or within 6 weeks of starting treatment if cancer treatment is successful.
The antigen CEA has been licensed by the Dendreon Corp for development as an Autologous Cellular Immunotherapy. Product candidates targeted at CEA are in preclinical development for the treatment of breast, lung, and colon cancer.
Antibodies to CEA are also commonly used in immunohistochemistry to identify cells expressing the glycoprotein in tissue samples. In adults, CEA is expressed only in cancer cells, especially adenocarcinomas, such as those arising in the colon, lung, breast, stomach, or pancreas. It can therefore be used to distinguish between these and other similar cancers. For example, it can help to distinguish between adenocarcinoma of the lung and mesothelioma, a different type of lung cancer which is not normally CEA positive. Because even monoclonal antibodies to CEA tend to have some degree of cross-reactivity, occasionally giving false positive results, it is commonly employed in combination with other immunohistochemistry tests, such as those for BerEp4, WT1, and calretinin.
The following is a list of human genes which encode carcinoembryonic antigen-related cell adhesion proteins:
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- Thomas SN, Zhu F, Schnaar RL, Alves CS, Konstantopoulos K (Jun 2008). "Carcinoembryonic antigen and CD44 variant isoforms cooperate to mediate colon carcinoma cell adhesion to E- and L-selectin in shear flow". J Biol Chem 283 (23): 15647–55. doi:10.1074/jbc.M800543200. PMC 2414264. PMID 18375392.
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- Duffy, M. J.; Van Dalen, A.; Haglund, C.; Hansson, L.; Klapdor, R.; Lamerz, R.; Nilsson, O.; Sturgeon, C. et al. (2003). "Clinical utility of biochemical markers in colorectal cancer: European Group on Tumour Markers (EGTM) guidelines". European journal of cancer (Oxford, England : 1990) 39 (6): 718–727. doi:10.1016/S0959-8049(02)00811-0. PMID 12651195.
- Maestranzi, S.; Przemioslo, R.; Mitchell, H.; Sherwood, RA. (Jan 1998). "The effect of benign and malignant liver disease on the tumour markers CA19-9 and CEA.". Ann Clin Biochem. 35 ( Pt 1): 99–103. PMID 9463746.
- Leong, Anthony S-Y; Cooper, Kumarason; Leong, F Joel W-M (2003). Manual of Diagnostic Cytology (2 ed.). Greenwich Medical Media, Ltd. pp. 51–52. ISBN 1-84110-100-1.
- Hammarström S (April 1999). "The carcinoembryonic antigen (CEA) family: structures, suggested functions and expression in normal and malignant tissues*1". Seminars in Cancer Biology 9 (2): 67–81. doi:10.1006/scbi.1998.0119. PMID 10202129.