H&E-stained lung sections from naive and challenged mice. Lung sections were fixed in 10% formalin 24 hours after the last OVA challenge. (A) naive wt mouse, naive CD34^−/− mouse, OVA-challenged wt mouse, and OVA-challenged CD34^−/− mouse. (B) Mean histologic scoring of peribronchial, perivascular, and parenchymal infiltration and epithelium damage (n=6). A score of 1 to 5 (0=no sign of disease, 5=profound inflammation) for each parameter was blindly attributed after analysis of 5 fields from each sample. Error bars represent SEM on mean score for each group.