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Many tumor cells produce antigens, which may be released in the bloodstream or remain on the cell surface. Antigens have been identified in most of the human cancers, including Burkitt lymphoma, neuroblastoma, malignant melanoma, osteosarcoma, renal cell carcinoma, breast carcinoma, prostate cancer, lung carcinomas, and colon cancer. A key role of the immune system is detection of these antigens to permit subsequent targeting for eradication. However, despite their foreign structure, the immune response to tumor antigens varies and is often insufficient to prevent tumor growth.
Tumor-associated antigens (TAAs) are relatively restricted to tumor cells, whereas tumor-specific antigens (TSAs) are unique to tumor cells. TSAs and TAAs typically are portions of intracellular molecules expressed on the cell surface as part of the major histocompatibility complex.
Suggested mechanisms of origin for tumor antigens include
Introduction of new genetic information from a virus (eg, human papillomavirus E6 and E7 proteins in cervical cancer)
Alteration of oncogenes or tumor suppressor genes by carcinogens, which either generate a novel protein sequence directly or induce accumulation of proteins that are normally not expressed or are expressed at very low levels (eg, ras, p53 )
Abnormally high levels of proteins that normally are present at substantially lower levels (eg, prostate-specific antigens, melanoma-associated antigens) or that are expressed only during embryonic development (carcinoembryonic antigens)
Uncovering of antigens normally buried in the cell membrane because of defective membrane homeostasis in tumor cells
Release of antigens normally sequestered within the cell or its organelles when tumor cells die
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