In the early 1980s, the discovery of cytolytic T cells (cytolytic T lymphocytes, CTLs) directed against an antigen presented on tumor cells was published. They can kill many invasive cells by recognizing the tumor specific antigens on the major histocompatibility complex (MHC) molecules. It is now generally agreed that although tumors express tumor antigens, they usually lack immunogenicity because of their inability to activate the immune response. The current view is that tumor cells are antigenic, but not immunogenic. If we can artificially activate the immune system against the tumor, it may be possible to eradicate the tumor. This approach is the basis of cancer immunotherapy.