CD4 and infection: To determine the cell types that caused the severe inflammation in aged mice, we isolated mononuclear cells on day 5 after A/PR8 infection from lungs of aged mice and stained cells with lineage defining markers, that is, CD8 and CD4 for T cell subsets, CD19 for B cells, CD11b for monocytes, Gr-1 for neutrophils and CD94 for NK cells.