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In the article are described some key mechanisms, used by an immune system to distinguish a tumor cell from a normal, healthy cell. According to bulk of the literature in the past decades, T lymphocytes are able to specifically recognise tumor cells, but the specific recognition of tumor cells does not result in rejection of a tumor tissue. Some proposed mechanisms, that would make an immune system tolerant to a tumor, are described. Accordingly to them, there is a possibility of some new therapeutic approaches, that would mobilise an immune system for an anti-tumor defence. Also natural killer cells and macrophages are able to distinguish tumor cells from normal, healthy cells and are also able to directly kill tumor cells. The nature of immune recognition by natural killer cells and macrophages is not known yet, because these cells do not express receptors for specific antigen recognition. Natural killer cells seem to be of a particular importance for a defence of an organism against tumors. The role of macrophages is less defined, because they are able to recognise and kill tumor cells, but on the other hand they are also able to promote tumor cells proliferation and the expansion of the tumor.