MPO and autoimmune disease: The anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitides (AAV) are a collection of relatively rare autoimmune diseases that are characterized by systemic necrotizing vasculitis of small vessels, particularly small arteries, arterioles, capillaries, and venules, and ANCAs, predominantly anti-myeloperoxidase and anti-proteinase 3, but little or no immune complex formation.[1] The pathological features of AAV are full-layer inflammation, necrosis, and granuloma formation in small vessels.