Many tumor cells produce antigens that can be released into the bloodstream or remain on the surface of cells. Antigens have been identified for most human malignant neoplasms, including such tumors as Berkitt's lymphoma, neuroblastoma, malignant melanoma, osteosarcoma, renal cell carcinoma, prostate cancer, lung cancer and colon cancer. The key role of the immune system is in recognizing tumor antigens, which makes it possible to determine the target for eradication in the future. However, despite the alien structure, the immune response to tumor antigens varies and is often insufficient to prevent tumor growth.
Tumor-associated antigens (TAA) have relative specificity for tumor cells, while tumor-specific antigens (TCA) are unique to malignant cells. TCA is usually expressed on the cell surface as part of the major histocompatibility complex.
The putative mechanisms for the origin of tumor antigens are as follows:
1) introduction of new viral genetic information (for example, human papillomavirus E6 and E7 proteins in cervical cancer);
2) damage of oncogenes or genes of tumor suppression by carcinogens, which leads to the generation of new proteins or induces the accumulation of proteins that under normal conditions are not expressed or expressed in very small quantities (ras, p53) \
3) abnormally high accumulation of proteins that are normally present in a small amount (for example, prostate-specific antigens, melanoma-associated antigens) or are expressed only during embryonic development (carcinoembryonic antigens);
4) expression of antigens normally hidden in the cell membrane as a result of disruption of membrane homeostasis by tumor cells; and 5) release of antigens normally isolated within the cell or its organelles during the death of the tumor cell.