Contrasting histopathology of asthma and chronic obstructive pulmonary disease (COPD). Contrasting histopathology of asthma and chronic obstructive pulmonary disease (COPD). A small airway from a patient who died from asthma and a similar sized airway from a patient with severe COPD are shown. There is an infiltration with inflammatory cells in both diseases. The airway smooth muscle (ASM) layer is thickened in asthma but only to a minimal degree in COPD. The basement membrane (BM) is thickened in asthma due to collagen deposition (subepithelial fibrosis) but not in COPD, whereas in COPD collagen is deposited mainly around the airway (peribronchiolar fibrosis). The alveolar attachments are intact in asthma, but disrupted in COPD as a result of emphysema. Images courtesy of J. Hogg (Vancouver, Canada). P.J. Barnes breathe 2011;7:229-238 ©2011 by European Respiratory Society