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Tumor antigen is an substance produced in tumor cells, i.e., it triggers an immune response in the host. Tumor antigens are useful in identifying tumor cells with and are potential candidates for use in cancer therapy. The field of cancer immunology studies such topics.
This classification, however, is imperfect because many antigens thought to be tumor-specific turned out to be expressed on some normal cells as well. The modern classification of tumor antigens is based on their molecular structure and source.
Accordingly, they can be classified as;
Other examples include tissue differentiation antigens, mutant protein antigens, oncogenic , cancer-testis antigens and vascular or stromal specific antigens. Tissue differentiation antigens are those that are specific to a certain type of tissue. Mutant protein antigens are likely to be much more specific to cancer cells because normal cells shouldn't contain these proteins. Normal cells will display the normal protein antigen on their MHC molecules, whereas cancer cells will display the mutant version. Some viral proteins are implicated in forming cancer (oncogenesis), and some viral antigens are also cancer antigens. Cancer-testis antigens are antigens expressed primarily in the of the testes, but also in fetal ovaries and the trophoblast. Some cancer cells aberrantly express these proteins and therefore present these antigens, allowing attack by T-cells specific to these antigens. Example antigens of this type are CTAG1B and MAGEA1.
Proteins that are normally produced in very low quantities but whose production is dramatically increased in tumor cells, trigger an immune response. An example of such a protein is the enzyme tyrosinase, which is required for melanin production. Normally tyrosinase is produced in minute quantities but its levels are very much elevated in melanoma cells.
Oncofetal antigens are another important class of tumor antigens. Examples are alphafetoprotein (AFP) and carcinoembryonic antigen (CEA). These proteins are normally produced in the early stages of embryonic development and disappear by the time the immune system is fully developed. Thus self-tolerance does not develop against these antigens.
Abnormal proteins are also produced by cells infected with , e.g. EBV and HPV. Cells infected by these viruses contain latent viral DNA which is transcribed and the resulting protein produces an immune response.
In addition to proteins, other substances like cell surface and may also have an abnormal structure in tumor cells and could thus be targets of the immune system.
| Alphafetoprotein (AFP) | Germ cell tumors
Hepatocellular carcinoma | |
| Carcinoembryonic antigen (CEA) | Occasional lung or breast cancer | |
| CA-125 | Ovarian cancer | |
| MUC-1 | Breast cancer | |
| Epithelial tumor antigen (ETA) | Breast cancer | |
| Tyrosinase | Malignant melanoma | normally present in minute quantities; greatly elevated levels in melanoma |
| Melanoma-associated antigen (MAGE) | Malignant melanoma | Also normally present in the testis |
| abnormal products of Ras subfamily, p53 | Various tumors |
Certain tumor antigens are thus used as . More importantly, tumor antigens can be used in cancer therapy as cancer vaccine.M Hareuveni, C Gautier, M Kieny, D Wreschner, P Chambon and R Lathe; Vaccination Against Tumor Cells Expressing Breast Cancer Epithelial Tumor Antigen; Proceedings of the National Academy of Sciences, Vol 87, 9498-9502, 1990.
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