Cancer cells start out as normal body cells, but they begin to grow out of control usually because of abnormal gene expression. Cells of the immune system are also susceptible to becoming different types of cancer cells such as leukemias and lymphomas. The immune system plays a major role in limiting the development of cancer cells, often before these cells have a chance to grow. Since the immune system responds to the environmental factors it encounters on the basis of discrimination between self and non-self, some cancer cells escape the immune system’s surveillance mechanism because the cells outer appearance (proteins and other molecules expressed on their surface) look normal, even though profound changes have occurred on the inside. In this way, these abnormal cells grow and multiply without triggering an immune response.
Scientists have begun to explore novel therapies to combat the different types of cancers that arise. Many of these experimental therapies are termed ‘Immunotherapy’ which is based upon the idea of stimulating the patient's immune system to attack the malignant tumor cells that are responsible for the disease. In spite of the fact that many cancer cells look ‘normal’ to the immune system, many kinds of cancer cells display unusual antigens that are either inappropriate for the cell type and/or its environment, or are only normally present during the organisms' development. The strategy used by scientists is to first identify these abnormal antigens expressed on cancer cells, and then trick the immune system into killing any cell that expresses these antigens.