Immunohistochemical staining of avian influenza virus antigens in lung tissue from a pig. Immunohistochemical detection of avian influenza virus (AIV) antigens in lung section from AIV infected pig euthanized PID 4, using goat anti-influenza A antibody and counterstaining with hematoxylin. AIV antigen positive (red) epithelial cells are seen in the alveolar septa (arrows). The area is partly consolidated. (Original magnification x20).