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A degeneration of a normal cell into a malignant cell is accompanied by development on the cell membrane of molecules specific for tumor cells-the tumor antigens. Such antigens stimulate the immune response in the body, and near the tumor cells cytotoxic T-lymphocytes, natural killers and macrophages appear, which bond to tumor cells and kill them. Both in vitro and in clinical experiments, however, a strengthening of the immune system in immunotherapy has been observed to stimulate tumor growth. The mechanisms of this paradoxical effect are unclear, but its existence is a restraining factor to immunotherapy. Another paradox in tumor development is the so-called state of tumor dormancy. This is a phenomenon whereby a small, clinically unidentifiable, number of tumor cells may exist in a body for a long time (months or years). A model describing these phenomena is presented.