KLRD1 and viral infectious disease: Since viral infection often results in reduced cell surface class I HLA expression, NK cell inhibitory signals will be dampened, allowing NK cells to lyse altered autologous target cells if activating receptors such as the natural cytotoxicity receptors (NCRs) NKp46, NKp44, NKp30, or NKG2D or nonclassical class I HLA-E-specific NCRs CD94/NKG2C and CD94/NKG2E are engaged (see Figure 2) [45, 70–75].