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One of the most challenging puzzles in medical science is the development of cancer. Understanding the process of malignant cell transformation could potentially help researchers reduce cancer morbidity. The mechanisms that cause cancer are frequently divided into (1) chemical substances, (2) viral infection, and (3) radiation (1). In all three mechanisms, the genetic code of the tumor cell is altered, which is seen in the following cellular characteristics: reduced apoptosis, uncontrolled cell growth ignoring tissue borders, metastasis via blood and lymph vessels, as well as neovascularization (2).
Genetic changes in tumor cells
The distinct genetic changes found in tumor cells are subdivided into 'gain of function' mutations, which mutate proto-oncogenes into oncogenes, or 'loss of function' mutations, which lead to inactivation of tumor-suppressive genes. During regular cell proliferation, spontaneous cell mutations are recognized by repair mechanisms, or, alternatively, apoptosis is induced. These repair mechanisms are permanently activated in preneoplastic diseases, where they enable temporary control of tumor growth. However, as even these repair mechanisms can fail, it is not possible to securely suppress the development of cancer. Differences in the phenotype or genotype of tumor cells and healthy tissue cells are the basis for immunological interventions with the aim to inhibit tumor growth. An immunologic approach has the potential to influence tumor cells or single subpopulations of the host immune system by targeted manipulation. Ideally, a tumor cell can be identified by its unique expression pattern of surface markers, which differs from that of the healthy originating cell tissue. By these means, the host immune system would be able to recognize and destroy all tumor cells that were not detected by the preceding standard oncologic therapy.
Adaptive immunity plays an especially important role because it produces a great variety of antigen-specific B cells and T cells by random recombination of DNA segments (somatic recombination). A specific immune response is induced if a clear difference between 'self' and 'nonself' is recognized, which is always the case in bacterial infections or allogeneic transplantation. In tumor cells, however, genetic mutations often lead to overexpression of surface markers, which are regularly found on healthy cells of the human body. Consequently, a clear identification of tumor cells as 'nonself' is difficult. The expression of completely new structures on the...