Diffuse pulmonary hemorrhage: evolutionary 'flaw' or consequence of evolutionary progress?

Diffuse alveolar hemorrhage (DAH), a serious and frequently life-threatening process, can occur in a wide variety of disorders, including mitral stenosis, systemic autoimmune diseases, conditions associated with anti-neutrophil cytoplasmic antibodies, anti-glomerular basement membrane antibody, inhaled toxins or infection, and drug allergies. These causes have no common denominator, but the end result, of capillary inflammation and hemorrhage, is the same. The capillaritis seen in DAH is present exclusively in the lung and is not part of a more generalized systemic vasculitis. In a recent article, I highlighted the central role of the neutrophils in this process, and suggested that in DAH the neutrophils present in the lung may sift the associated immune complexes and transfer them out of the vascular system into the alveoli. In this article, we document the inflammation seen during the remission period and during the acute lung bleeding of DAH in 2 patients. Because DAH is a common finding in a variety of pathological conditions, we then explore the possible evolutionary mechanisms behind DAH.