Vasculitis is inflammation of vessel walls. It has many causes, although they result in only a few histologic patterns of vascular inflammation. Vessels of any type in any organ can be affected, a fact that results in a wide variety of signs and symptoms. These protean clinical manifestations, combined with the etiologic nonspecificity of the histologic lesions, complicate the diagnosis of specific forms of vasculitis. This is problematic because different vasculitides with indistinguishable clinical presentations have very different prognoses and treatments. For example, a patient with purpura, nephritis, and abdominal pain caused by Henoch–Schönlein purpura usually has a good prognosis . . .