Antineutrophil cytoplasmic antibodies (ANCA) are a type of antibody produced by the immune system that mistakenly targets and attacks certain proteins in neutrophils, a type of white blood cell. These antibodies are often associated with autoimmune diseases, where the body's immune system erroneously attacks its own tissues. The presence of ANCA can be detected through blood tests and is commonly linked to conditions such as granulomatosis with polyangiitis and microscopic polyangiitis. Identifying these antibodies helps healthcare providers diagnose and manage these autoimmune disorders effectively.