Interstitial Lung Disease
Inflammation and progressive fibrosis thicken and stiffen the interstitium between the alveolus and its capillary, so the lung becomes small and stiff rather than obstructed, impairing oxygen diffusion out of proportion to airflow and producing the restrictive, breathless, dry-cough picture that distinguishes it from obstructive airway disease.
First principles
The disease is in the wall between air and blood, not the airway
Interstitial lung disease is a group of conditions (idiopathic pulmonary fibrosis, connective tissue disease-associated ILD, hypersensitivity pneumonitis, occupational exposures such as asbestosis, and others) united by a shared target: the interstitium, the thin connective tissue space between the alveolar epithelium and capillary endothelium across which oxygen normally diffuses in a fraction of a second. Chronic inflammatory or fibrotic injury thickens this space with collagen and scar tissue. Because the airways themselves are largely untouched, airflow is not obstructed: the lung instead becomes stiff and shrunken, which is the fundamental physiological difference from asthma, COPD or bronchiectasis.
Educational content pending clinical review. Not medical advice.