MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: ANCA-associated vasculitis (AAV) is a group of primary vasculitides, characterised by the presence of autoantibodies against the neutrophil antigens proteinase 3 (PR3-ANCA) or myeloperoxidase (MPO-ANCA) and leucocyte infiltration of small blood vessel walls, resulting in vascular damage and organ dysfunction.