**3. Pathology**

Perivascular cellular infiltration is a common histological finding in many disease entities, but for a definitive diagnosis of vasculitis, the presence of vascular damage, particularly in the form of fibrinoid degeneration, is necessary.

Vasculitis may involve blood vessels of varying calibers and this feature forms the basis of a useful pathological classification of vasculitis. An infiltrate, composed of a variety of cell types, like neutrophils, lymphocytes, and histiocytes may invade the vessel wall and the surrounding tissue. Extravasation of red cells is a prominent feature in many vasculitides. Granulomatous inflammation with giant cell formation is a characteristic finding in some types.
