Idiopathic Pulmonary Hemosiderosis

The pulmonary capillaries are generally assumed to be the source of bleeding in idiopathic pulmonary hemosiderosis, but the nature of the capillary defect is unknown. Electron microscopic study of a lung biopsy from a patient with idiopathic pulmonary hemosiderosis showed evidence of widespread abnormalities of the pulmonary capillaries. The endothelium was swollen, and dense deposits of protein were present in the capillary basement membranes. The morphologic similarity of these deposits to those in glomeruli in “immune complex” nephritis and the decreased serum complement in this patient suggested an immunologic mechanism for the intrapulmonary hemorrhage.