Double immunohistochemical staining of swine influenza virus antigens and epithelial cells. Lung sections from swine influenza virus (SIV) H1N1 infected pig euthanized PID 4. Double immunohistochemical staining using polyclonal anti-influenza and anti-cytokeratin antibodies demonstrating SIV antigen positive cells (clear blue cells) and epithelial cells (clear red cells). Double positive cells (SIV antigen positive epithelial cells) are purple (counterstained with hematoxylin). A: Demonstration of double positive bronchioles (arrowhead). B: Demonstration of both double stained alveolar type I (arrow) and II epithelial cells (arrowhead). (Original magnifications A: x10, B: x40).