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signal to the cell to stop dividing. In germ cells, adult stem cells, and cancer cells the telomerase gene is “switched on,” producing an enzyme that rebuilds the telomeres. Thus, like germ cells, the cancer cell becomes immortal and able to divide indefinitely without losing its telomeres.Immunologists recognized that many cancers expressed antigens on the cell surface that were not normally expressed on the surface of non–cancer cells from the same tissue. Two hypotheses arose from these observations. First, immunologists predicted that many developing malignancies were suppressed by an efficient immune surveillance system by which cells expressing tumor-associated antigens were identified, and the resultant immune response destroyed developing malignant cells. Second, when tumors do develop, their tumor-associatedantigens could be targets of effective immunotherapy. Immunotherapy could be either active by immunization with tumor antigens to elicit or enhance the immune response against a particular cancer, or passive by injecting the cancer patient with antibodies or lymphocytes directed against the tumor-associated antigens.A number of viruses can cause cancer. Human cervical cancer is caused by papillomavirus infection. Kaposi sarcoma is caused by infection with HHV8, a member of the Herpesviridae family. Chronic hepatitis infection with HBV or HCV is the leading cause of liver cancer.Defects in the immune system increase the risk of viral-associated cancers but have a minimal effect on the risk of other cancers.Active inflammation predisposes to cancer by stimulating a wound-healingresponse that includes proliferation and new blood vessel growth.Epstein-Barr virus (EBV) and HHV8 are members of the Herpesviridae family.44 More than 90% of adults have been infected with EBV, usually as children and without symptoms. EBV infection during adolescence may cause infectious mononucleosis. The virus infects B lymphocytes and stimulates their proliferation; the infection usually remains latent throughout the individual’s life. In individuals who are immunosuppressed because of HIV infection or because of drugs given for an organ transplant, persistent EBV infection can lead to the development of B-cell lymphomas. Development of B-cell lymphomas in persons with organ transplants is known as post-transplant lymphoproliferative disorder (PTLD). One effective therapy for PTLD is, if possible, to decrease or stop the administration of immunosuppressant drugs and allow the immune system to attack the virus. Epstein-Barr virus - EBV infection also is associated with Burkitt lymphoma in areas of endemic malaria and with nasopharyngealcarcinoma, a cancer endemic in Chinese populations in Southeast Asia
Vaccinations can prevent hepatitis B–associated liver cancer and many human papillomavirus-caused cervical cancers.