Anti-neutrophil cytoplasmic autoantibodies (ANCA) are found in the blood of most patients with pauci-immune necrotizing vasculitis and crescentic glomerulonephritis. This study shows that ANCA can activate neutrophils in vitro, leading to the release of reactive oxygen species (ROS) and granule contents. Both ANCA phenotypes—cytoplasmic-patterned ANCA and myeloperoxidase-specific ANCA—induce neutrophil activation. ANCA sera and purified immunoglobulins significantly increase ROS release compared to controls. ANCA also induce the release of primary granule contents, which is enhanced by tumor necrosis factor (TNF) priming. Flow cytometry shows that TNF-primed neutrophils have ANCA antigens on their surface, allowing interaction with ANCA. These findings suggest a role for ANCA in the pathogenesis of necrotizing vasculitis and glomerulonephritis. ANCA-induced release of toxic oxygen radicals and granule enzymes from cytokine-primed neutrophils may mediate vascular inflammation. ANCA react with constituents of neutrophil primary granules and monocyte lysosomes. Two types of ANCA are recognized: C-ANCA, which reacts with proteinase 3, and MPO-ANCA, which reacts with myeloperoxidase. The study used in vitro experiments to measure ROS and granule release from ANCA-stimulated neutrophils. Results showed that ANCA-induced ROS release was significantly higher than in controls. ANCA also caused degranulation of primary granules, which was enhanced by TNF priming. Flow cytometry confirmed that TNF-primed neutrophils expressed MPO on their surface, indicating ANCA antigen interaction. These findings support the hypothesis that ANCA contribute to the pathogenesis of necrotizing vasculitis and glomerulonephritis by activating neutrophils and releasing toxic substances. The study was supported by grants from the National Institutes of Health and the Thomas R. Arthur Trust.Anti-neutrophil cytoplasmic autoantibodies (ANCA) are found in the blood of most patients with pauci-immune necrotizing vasculitis and crescentic glomerulonephritis. This study shows that ANCA can activate neutrophils in vitro, leading to the release of reactive oxygen species (ROS) and granule contents. Both ANCA phenotypes—cytoplasmic-patterned ANCA and myeloperoxidase-specific ANCA—induce neutrophil activation. ANCA sera and purified immunoglobulins significantly increase ROS release compared to controls. ANCA also induce the release of primary granule contents, which is enhanced by tumor necrosis factor (TNF) priming. Flow cytometry shows that TNF-primed neutrophils have ANCA antigens on their surface, allowing interaction with ANCA. These findings suggest a role for ANCA in the pathogenesis of necrotizing vasculitis and glomerulonephritis. ANCA-induced release of toxic oxygen radicals and granule enzymes from cytokine-primed neutrophils may mediate vascular inflammation. ANCA react with constituents of neutrophil primary granules and monocyte lysosomes. Two types of ANCA are recognized: C-ANCA, which reacts with proteinase 3, and MPO-ANCA, which reacts with myeloperoxidase. The study used in vitro experiments to measure ROS and granule release from ANCA-stimulated neutrophils. Results showed that ANCA-induced ROS release was significantly higher than in controls. ANCA also caused degranulation of primary granules, which was enhanced by TNF priming. Flow cytometry confirmed that TNF-primed neutrophils expressed MPO on their surface, indicating ANCA antigen interaction. These findings support the hypothesis that ANCA contribute to the pathogenesis of necrotizing vasculitis and glomerulonephritis by activating neutrophils and releasing toxic substances. The study was supported by grants from the National Institutes of Health and the Thomas R. Arthur Trust.