Histopathology and immunohistochemistry examination of H5N1 antigens. The survival chicks were sacrificed, and the lungs were removed and fixed in buffered formalin. Hematoxylin and eosin staining were performed on paraffin sections as previously described. For the immunohistochemical analyses, a monoclonal antibody against influenza A was incubated on the sections. A FITC-conjugated secondary antibody was incubated. Severe inflammation (A) and viral antigens (B) were observed in the lung of chicks housed in the untreated-filter box. In contrast, obvious histopathological legions (C) and viral antigens (D) were not detected in the lungs of chicks housed in antibody filter boxes. Scale bars represent 100 μm.