The clinical experiments of William Coley in the 1890s and the work of Prehn and Main in the 1950s firmly demonstrated the presence of an immune response against tumor antigens, which could potentially be used in the treatment of cancer. More recent concepts in carcinogenesis including the viral etiology of some malignancies, the presence of mutated onco-proteins and the over-expression of certain normal antigens, have further supported the concept that an antigen specific immune response can be generated to control tumor growth. However, it has also become evident that tumor cells have developed sophisticated mechanisms to evade the immune response, allowing them to proliferate and invade normal tissues. The mechanisms by which tumor cells escape the immune system are still a matter of active research and debate, with possible explanations ranging from the inability of the immune system to recognize tumor antigens (tumor ignorance) to the gradual deterioration of the immune