Pathophysiology
Clinical meaning
Diffuse alveolar hemorrhage (DAH) results from injury to the alveolar-capillary basement membrane, allowing red blood cells to flood the alveolar spaces. The pathology involves pulmonary capillaritis (neutrophilic infiltration and necrosis of capillary walls), bland hemorrhage (without inflammation), or diffuse alveolar damage. Hemosiderin-laden macrophages accumulate within 48-72 hours, and repeated episodes lead to pulmonary fibrosis. Autoimmune vasculitides (GPA, MPA, anti-GBM disease) are the most common causes, with immune complex deposition triggering complement activation and capillary destruction.