Chronic lymphocytic leukemia (CLL) is the most frequent leukemia in elderly patients and therefore of high clinical relevance. Although the first report of defects in tumor response by NK cells in CLL were published 35 years ago, the mechanism underlying immune evasion is not sufficiently known. Besides, the effects of direct cell to cell interaction between CLL and NK cells are also not well described yet. Thus, my main aim is to further characterise the immunosuppressive effect on cytotoxic lymphocytes, mainly NK cells, via direct contact to CLL cells or indirect via the cytokine secretion. A deep understanding of the mechanism by which NK cell cytotoxicity is reduced in CLL will foster the development of possible new combinations of immunotherapies, ultimately resulting in an improved immune response against malignant B-cells.