Anti-neutrophil cytoplasmic antibodies (ANCAs) are a group of mainly IgG antibodies against antigens in the cytoplasm of neutrophil granulocytes (the most common type of white blood cell) and monocytes. They are detected as a blood test in a number of autoimmune disorders, but are particularly associated with systemic vasculitis, so called ANCA-associated vasculitides. ANCA were originally shown to divide into two main classes: c-ANCA and p-ANCA, based on the pattern of staining on ethanol-fixed neutrophils and the main target antigen. p-ANCA show a perinuclear staining pattern. The most common p-ANCA target is myeloperoxidase (MPO), a neutrophil granule protein whose primary role in normal metabolic processes is generation of oxygen radicals. c-ANCAs show a diffusely granular, cytoplasmic staining pattern. The most common protein target being proteinase 3 (PR3). PR3 is the most common antigen target of ANCA in patients with Wegeners granulomatosis. Other antigens may also occasionally result in a c-ANCA pattern. Classically, c-ANCA is associated with Wegener’s granulomatosis; p-ANCA is associated with microscopic polyangiitis and focal necrotising and crescentic glomerulonephritis. However, in recent years ANCA targeted against other autoantigens have been identified, including Churg-Strauss syndrome. See Myeloperoxidase (MPO) Antibodies, Serum; and Proteinase 3 (PR3) Antibodies, Serum