The design of the human body is such that it can fight against infections and cancer cells and can kill them. An essential feature of the human immune system is recognizing and distinguishing between its cells (known as 'self') and foreign cells like bacteria and viruses (known as 'non-self'). Every cell has proteins related to their origin and functioning environment, which are called antigens, which serve to differentiate between self or non-self by the immune system. As a component of the immune system, antibodies or immunoglobulins are proteins produced by a type of immune cells called B Lymphocytes, which get converted into plasma cells eventually. These antibodies are mostly produced during exposure to infection or by binding to antigens, and their job is to neutralize or eliminate non-self-entities entering the body. Antibodies are also produced via vaccination by either exposure to killed antigens or direct inoculation of already formed antibodies in vitro.
The immune system's failure to recognize 'self' is called autoimmunity, in which the immune system attacks some tissues of the human body and destroys it. In autoimmunity, there is the formation of antibodies against self-antigens called autoantibodies. These autoantibodies form against one type of cell to which they bind and destroy.
Anti-neutrophil cytoplasmic antibodies are specific antibodies formed against cytoplasmic granules (antigens) of polymorphonuclear neutrophil granulocytes (PMNs). These autoantibodies are present in ANCA-associated small-vessel vasculitides. The applied use of ANCA lab test, which detects ANCA autoantibodies, is used in the diagnosis of a few vasculitis diseases, mainly pauci-immune small vessel vasculitides granulomatosis with polyangiitis (GPA or Wegener granulomatosis), microscopic polyangiitis (MPA), and to lesser extent eosinophilic granulomatosis with polyangiitis (EGPA or Churg Strauss syndrome) and anti-GBM disease.