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Natural killer (NK) cells are effector lymphocytes of innate immunity needed to protect against stressed cells and to destroy tumor cells and virus-infected cells. These cells play an important role in the immune surveillance of malignant cells, preventing their uncontrolled growth. Natural killer cells recognize target cells directly through their receptors, which bind to various determinants on the surface of the target cell. The receptor-ligand (secretory or membrane-bound) interaction between the NK cells and the target cells determines NK's cell activity. The use of monoclonal antibodies in tumor therapy has increased significantly in the recent years. These antibodies are intended to block inhibitory receptors (immune checkpoint inhibitors) expressed by immune cells or to block their ligands expressed by tumor cells. Examples of such immune checkpoint molecules are the following receptors: cytotoxic T lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1) and others. This blocking inhibits tumor growth by enabling immune system reactivation. The advances in cellular immunobiology that have provided the establishment of blocking monoclonal antibodies (ipilimumab, nivolumab, etc.) and increased NK cell proliferative activity are promising therapies for neoplasms.