What is ANCA (Anti neutrophil cytoplasmic antibodies)?

Antineutrophil cytoplasmic antibodies (ANCA)

Antineutrophil cytoplasmic antibodies (ANAC) are IgG antibodies directed against the cytoplasmic constituents of granulocytes (neutrophil). They are useful in diagnosis and monitoring of systemic vasculitis although those may be found in other conditions such as autoimmune liver disease, malignancy, infection (bacterial and human immunodeficiency virus, HIV), inflammatory bowel disease, RA, SLE and pulmonary fibrosis. 

Immunofluorescence study describes two common patterns of those antibodies; cytoplasmic fluorescence (c-ANCA) caused by the antibodies to proteinase-3(PR3), and perinuclear fluorescence (p-ANCA) caused by the antibodies to myeloperoxidase (MPO) and few other proteins such as lactoferrin, elastase, lysozyme and cathepsin G. 

c-ANCA: There's are some correlation between the c-ANCA level and disease activity. It more specifically (positive in>90%) found in Wegener's granulomatosis. Also found in microscopic polyangiitis (positive in 40% cases)

p-ANCA: It is heterogeneous. Can't be used for monitoring disease activity. One subset binds with MPO and found in idiopathic crescentic glomerulonephritis (positive in 80% patients), microscopic polyangiitis (MPA)- positive in 50-70% cases, Churg-Strauss syndrome (positive in 60%) and Wegener's granulomatosis (in 25%). 

Other subsets of p-ANCA bind with lactoferrin, elastase, lysozyme and cathepsin G. They are found in chronic active hepatitis (CAH), sclerosing cholangitis, ulcerative colitis (UC), SLE, mixed connective tissue disease, renal tubular acidosis (RTA) and cryptogenic fibrosing alveolitis (CFA).