Age-specific differences in leukocyte populations in the lung but not lymph node following SARS-CoV infection. (A) The number of total lung leukocytes was calculated per gram of tissue in standardized collected samples from mock- and SARS-CoV infected animals at 5 and 10 days post infection (d.p.i.). Samples were taken from the proximal portion of the right caudal lobe adjacent to the region used for cytokine analysis (Figures 3–4). Flow cytometry was used to further characterize the major leukocyte populations in the lung mucosa. The percent frequency of total lung leukocytes are indicated for (B) macrophages (CD68 + HLADR+), (C) dendritic cells (DCs; CD68-HLADR + CD11c+), (D) CD8 T lymphocytes (CD3 + CD8+) and (E) B cells (CD3-CD20+). (F) The number of total cells collected from the tracheobronchial lymph node (LN) tissue was quantitated for mock- and SARS-CoV infected animals at 5 and 10 d.p.i.. The percent frequency of total lymph node cells were determined for (G) CD8+ T cells and (H) B cells. Graphs represent average values (+/−SE). The gating strategies for flow cytometric analysis are shown in Additional file 1: Figures S1 and Additional file 2: Figure S2 and percent frequencies summarized in Additional file 3: Table S1. *p < 0.05, **p < 0.01 in unpaired student T-test comparing age groups. +p < 0.05 for comparing infection time points.