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Immunopathological changes in dermatomyositis sequence

Immunopathologic changes beginning with activation of complement and formation of C3 by antibodies (Y) against endothelial cells. Activated C3 leads to formation of C3b, C3bNEO, and MAC that cross the endothelial cell wall to the endomysial capillaries (A). Deposition of MAC leads to destruction and reduced number of capillaries, with ischemia or microinfarcts prominent in the periphery of the fascicle (B). A smaller than normal number of capillaries with dilated diameter remains and perivascular atrophy ensues (C). The complement-fixing antibodies (Y), B cells, CD8+ T cells, and macrophages traffic to the muscle. (Contributed by Dr. Marinos Dalakas.)