CD4 and infection: Compared to Rag1‐KO mice that received either PBS or non‐manipulated naïve CD4+ T cells from old mice, Rag1‐KO mice that received mito‐transferred naïve CD4+ T cells from old mice had a significant delay in IAV‐induced morbidity (weight loss) by day 8 post‐infection (Figure 6B).