Vasculitis is an inflammation of the blood vessels. It happens when the body's immune system attacks the blood vessel by mistake. The cause is often unknown. Vasculitis can affect arteries, veins and capillaries. Arteries are vessels that carry blood from the heart to the body's organs. Veins are the vessels that carry blood back to the heart. Capillaries are tiny blood vessels that connect the small arteries and veins.
MedlinePlus - Vasculitis