Double immunohistochemical staining of avian influenza virus antigens and epithelial cells. Lung sections from avian influenza virus (AIV) infected pig, euthanized PID 4. Double immunohistochemical staining using polyclonal anti-influenza and anti-cytokeratin antibodies (counterstaining with hematoxylin) demonstrating AIV antigen positive cells (clear blue cells) and epithelial cells (clear red cells). Double positive cells (AIV antigen positive epithelial cells) are purple. A: Demonstration of double positive alveolar epithelial cells (arrowhead). B: Greater magnification than A, demonstration of double stained alveolar type II epithelial cell (arrowhead). (Original magnifications A: x10, B: x40).