Antineutrophil cytoplasmic antibodies (ANCA) are a group of autoantibodies produced when a person's immune system mistakenly targets and attacks its own neutrophil proteins. Two of the most commonly targeted proteins are myeloperoxidase (MPO) and proteinase 3 (PR3). This results in the production of antibodies to MPO and/or PR3. The ANCA blood test detects the presence or absence of these autoantibodies. Antineutrophil cytoplasmic antibodies may be present in a variety of autoimmune disorders that cause inflammation and damage to blood vessels throughout the body (systemic vasculitis). Vasculitis can cause tissue and organ damage due to the narrowing and obstruction of blood vessels and the subsequent loss of blood supply. It can also produce areas of weakness in blood vessel walls, known as aneurysms, which have the potential to rupture.