Anti-neutrophil cytoplasmic antibodies (ANCA) are a type of antibody produced by the immune system that mistakenly targets proteins in neutrophils, a type of white blood cell. These antibodies can be involved in various autoimmune diseases, leading to inflammation and damage in blood vessels and organs. The presence of ANCA is often associated with conditions such as granulomatosis with polyangiitis and microscopic polyangiitis. Testing for ANCA can help diagnose these diseases, as their levels may indicate the severity of the condition and guide treatment options.