The immune system is generally able to recognize tumor cells and prevent tumor disease. Typically, tumor cells are rendered harmless by so-called killer cells (cytotoxic T-lymphocytes).

However, it can be fatal when tumor cells develop mechanisms which make it impossible for killer cells to fight the tumor. Tumor cells masquerade themselves in a certain way, and thus the cytotoxic T-lymphocytes can no longer recognize them as such.
Certain tumor cells produce the enzyme indoleamine 2,3 dioxygenase (IDO) in excess. Thereby tumor antigens are not “presented” any longer, which means that the immune system cannot recognize tumor cells as foreign anymore.
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