Together, innate and adaptive immune responses are capable of recognizing and destroying cancer. Natural killer cells and gamma-delta T cells are capable of specifically recognizing and killing tumor cells. Tumor antigens have been identified from many cancers, and cancer patients generate CD4+ and CD8+ T lymphocytes and B lymphocytes specific for these antigens. Tumor-specific lymphocytes are found infiltrating tumors and in the peripheral blood of cancer patients. The presence of brisk tumor infiltrating lymphocytes in primary melanoma portends a survival advantage compared to tumors where tumor infiltrating lymphocytes are absent. In some instances, the immune system can cause spontaneous regression of tumors. For example, partial regression is a common finding in primary melanoma lesions, and there are rare reports of complete regression of metastatic melanoma. The importance of the immune system in preventing cancer is reflected in the increased frequency of malignancies in immunosuppressed and immunodeficient patients. Under the selective pressure of the immune response, tumors evolve to escape immune-mediated destruction.
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