Myeloperoxidase Antibody (MPO) – Autoimmune vasculitis diseases are characterized by abnormal immune responses that result in inflammation and necrosis of blood vessels. The immune dysfunction may be triggered by infection, autoimmune disease, or exposure to a drug; often the cause is unknown. ANCA-associated vasculitis diseases are often characterized by the size of the blood vessels involved. The diseases present with diverse clinical features and are often rapidly progressive, causing irreversible injury to the vessels of the organs affected, such as the kidneys and lungs.