Nks cells originate from the bone marrow and represent about 5 to 10 % of the lymphocytes in the blood. They belong to the first line of defenses against viruses and thus reprensent an important part of the innate immune system. Upon activation, NK cells lyse the infected cells or release cytokines such as interferon gamma, which activates macrophages and T-cells. Theses functions help to control infection during initial hours allowing the adaptive immune response to develop. Many viruses have evolved strategies to inhibit NK cells activity to allow efficient infection. For example, a lot of herpesvirus proteins function to evade NK cell activation. In addtion to the attenuation of the adaptive response, these proteins allow persistent infection within the host.
Table. Viruses modulating the activity of NK cells.
| Virus | Family | Viral protein | Ation | References |
| HCMV | herpesviridae, Cytomegalovirus | US2, US11 | Causes degradation of MHC class I by proteasomes | 
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| HCMV | herpesviridae, Cytomegalovirus | US3, US10 | Retention of MHC class I in ER | |
| HCMV | herpesviridae, Cytomegalovirus | US6 | Prevents loading of peptide into MHC I molecules | |
| HCMV | herpesviridae, Cytomegalovirus | gpUL40 | Downregulates MHC I molecules from cell surface | |
| HIV | Retroviridae, Lentivirus | Nef | Accelerates HLA-A and B endocytosis | |
| MCMV | herpesviridae, Cytomegalovirus | m04 | Binds MHC I on cell surface | |
| MCMV | herpesviridae, Cytomegalovirus | m06 | Targets MHC class I for degradation | |
| MCMV | herpesviridae, Cytomegalovirus | m152 | Downregulates MHC I form cell surface | |
| MOCV | Poxviridae, Molluscipoxvirus | MC53L ,MC54L | Binds IL-18, blocks INF-gamma production | |
| HCV | Flaviviridae, Hepacivirus | E2 | Binds CD81 receptor on NK cells inhibition killing and cytokine production | |
| KSHV | Herpesviridae, Rhadinovirus | K3, K5 | Induces MHC class I endocytosis | |
| HHV-1 | Herpesviridae, Simplexvirus | ICP47 | Prevents loading of peptides into MHC I | |