Many tumor cells produce cancer-associated or cancer-specific molecules, usually peptides, which may be released in the blood or displayed on the cell surface. Any molecule capable of being recognized by the immune system and triggering an immune response is considered antigenic. Cancer-associated or cancer-specific antigens have been identified in most of the human cancers, including Burkitt lymphoma , neuroblastoma , melanoma , osteosarcoma , renal cell cancer , breast cancer , prostate cancer , lung cancer , and colon cancer . A key role of the immune system is detection of these antigens to permit subsequent targeting for eradication. However, the immune response to tumor antigens varies and is often insufficient to prevent tumor growth (see also Host Response to Tumors ).