definition according to Dorlands for antineutrophil cystoplasmic antibodies:

An autoantibody to cytoplasmic consituents of monocytes and neutrophils, found in increased amounts in some types of vasculitis.

An autoantibody is (according to Dorlands) an antibody directed against a self antigen.

If my understanding of rheumatology/rheumatoid arthritis is accurate, it happens when the body starts attacking itself (I think autoantibody would fit here). Hope this helps.