Function of T and natural killer (NK) lymphocytes is tightly controlled by the balance of activating and inhibitory signals. NK receptors belong to different families: KIRs ("Killer cell Immunoglobulin-like Receptor") and ILTs ("Immunoglobulin-like Transcript"), mainly inhibitory which binds to HLA class I alleles; C-type lectin NK receptors such as CD94/NKG2A which is inhibitory and binds to HLA-E; NCR ("Natural Cytotoxicity Receptors") which directly activate NK cells. These include molecules NKp30, NKp44, NKp46 et NKG2D. Cellular stress (viral and bacterial infections, tumours) may modulate NK function by different mechanisms: decrease in HLA class I molecules expression resulting in the lack of engagement of the inhibitory receptors and ultimately NK cell activation; modulation of CD94/NKG2A inhibitory function through expression of peptides presented by HLA-E as for instance from heat shock proteins; NK activation through NCR expression. Among these, NKG2D is an activating receptor expressed by NK cells and subsets of alphabeta and gammadelta and T cells. Major NKG2D ligands in humans are MIC ("MHC class I related") molecules which are stress-inducible during a viral (CMV) or bacterial infection (M. tuberculosis, E. coli). They may also be expressed by tumors. Therefore, they could play a role in activating NK and/or T lymphocyte responses in these conditions.