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Antineutrophil cytoplasmic antibodies (ANCA) are predominantly IgG autoantibodies directed against constituents of primary granules of neutrophils and monocytes lysosomes. Although several antigenic targets have been identified, those ANCA directed to proteinase 3 or myeloperoxidase are clinically relevant, whereas the importance of other ANCA remains unknown. Both are strongly associated with small vessel vasculitides, the ANCA-associated vasculitides, which include Wegener's granulomatosis, microscopic polyangiitis, and Churg-Strauss syndrome, and the localised forms of these diseases (eg, pauci-immune necrotising and crescentic glomerulonephritis). ANCA is a useful serological test to assist in diagnosis of small-vessel vasculitides. 85-95% of patients with Wegener's granulomatosis, microscopic polyangiitis, and pauci-immune necrotising and crescentic glomerulonephritis have serum ANCA. ANCA directed to either proteinase 3 or myeloperoxidase are clinically relevant, yet the relevance of other ANCA remains unknown. Besides their diagnostic potential, ANCA might be valuable in disease monitoring. In addition, data seem to confirm the long-disputed pathogenic role of these antibodies. There is increasing evidence that myeloperoxidase-ANCA are directly involved in the pathogenesis of necrotizing vasculitis. This is less clear for proteinase 3-ANCA, markers for Wegener's granulomatosis. With respect to proteinase 3-ANCA, complementary proteinase 3, a peptide translated from the antisense DNA strand of proteinase 3 and homologous to several microbial peptides, may be involved in induction of proteinase 3-antineutrophil cytoplasmic autoantibodies.

Churg-Strauss syndrome (CSS) is a rare systemic vasculitis occurring in patients with a history of asthma or allergy and eosinophilia. Although lungs, skin, and peripheral nervous system are the most common sites of involvement, many other organs, including heart, kidneys, and gastrointestinal tract, can be affected. CSS is an eosinophil-mediated disease, but its pathophysiology is not fully known. There are no specific laboratory tests for CSS. The main laboratory characteristics are peripheral blood eosinophilia, elevated serum IgE levels, and the presence of an inflammatory syndrome. Antineutrophil cytoplasmic antibodies directed against myeloperoxidase are positive in approximately one-half of patients. The diagnosis of CSS rests on the association of the clinical features of the disease with its histologic hallmarks. The biopsy of affected tissues may show a spectrum of histologic changes, including leukocytoclastic or necrotizing vasculitis, eosinophilic tissue infiltration, vascular and extravascular granulomas. The involved vessels are small arteries, veins, arterioles, venules, and capillaries. Corticosteroids, sometimes in combination with a cytotoxic agent, such as cyclophosphamide, are the most effective treatment of CSS. Intravenous immunoglobulins or interferon-alpha may be useful for patients refractory to conventional treatment.