Immunohistochemical staining for Activin-A in small airway of human lung (A). Detail of the airway epithelium (B). Isotype control staining is presented in (C) and (D). Activin-A is strongly expressed in the airway epithelium and smooth muscle layer. The mononuclear cell infiltrate also expresses Activin-A. A mild expression is also observed the connective tissue of the lamina propria and adventitia.