On day 5 post-infection, cytotoxic activity of NK cells is markedly up-regulated in both splenic and pulmonary tissues of severe combined immunodeficiency(SCID) mice, indicating that NK cells are rapidly activated at the onset of pathogen invasion., which lack T and B lymphocytes yet retain functional NK cells, depletion of NK cells with anti-asialo-GM1 antibody results in a significant reduction in pulmonary IFN-γ levels, providing direct evidence that NK cells are an important IFN-γ source [69]. This evidence concerns the gene IFNG and infection.