CD4 and infection: We examined the NK cell and the virus-specific CD8+ and CD4+ T cell responses in the lungs of influenza virus−infected WT and NKCKD mice 5 and 7 days p.i. Assessment of the percentage and absolute number of these lymphocyte subpopulations after influenza virus infection showed that a protective response to the infection within the first 5 days directly correlated with NK cell expansion (Fig 5A–5F), and not that of virus-specific CD4+ or CD8+ T cells.