What is the diagnosis for Wegener's Granulomatosis
(Granulomatosis with Polyangiitis)?

No one knows the cause of Wegener's granulomatosis. It is thought to be an autoimmune disorder in which the body's defense system attacks itself and destroys normal body tissue. Normally, the body's immune system acts as a defense against outside invaders. With an autoimmune disorder, something triggers an immune response causing the body to react against its own tissue. The result is that tissue can be damaged. With Wegener's granulomatosis, the organ systems that are attacked include the respiratory tract and kidneys.

Source: http://www.emedicinehealth.com