Many tumor cells produce antigens, which may be released in the bloodstream or remain on the cell surface. Any molecule capable of being recognized by the immune system is considered an antigen. Antigens have been identified in most of the human cancers, including Burkitt lymphoma, neuroblastoma, melanoma, osteosarcoma, renal cell carcinoma, breast cancer, prostate cancer, lung carcinoma, 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 (see also Host Response to Tumors).