In stark contrast, the remaining 12/27 solitary lesions in primary infected mice lacked this necrotic core and instead contained tightly aggregated clusters of antigen-bearing macrophages (CD68+, purified protein derivative [PPD]+), which were surrounded by intact alveolar epithelium (Fig. 1, D–F), consistent with alveolitis. This evidence concerns the gene CD68 and hypersensitivity pneumonitis.