This test looks for antineutrophil cytoplasmic antibodies (ANCA) in your blood. Antibodies are proteins that your immune system makes to fight foreign substances like viruses and bacteria. But ANCAs attack healthy cells known as neutrophils (a type of white blood cell) by mistake. This can lead to a disorder known as autoimmune vasculitis. Autoimmune vasculitis causes inflammation and swelling of the blood vessels.