Vasculitis is an inflammation of the blood vessels. Inflammation is a condition in which tissue is damaged by blood cells entering the tissues. These are mostly white blood cells which circulate and serve as our major defense against infection. Ordinarily, white blood cells destroy bacteria and viruses. However, they can also damage normal tissue if they invade it.
Several things can happen to an inflamed blood vessel.
If it is a small vessel, it may break and produce tiny areas of bleeding in the tissue. These areas will appear as small red or purple dots on the skin. If a larger vessel is inflamed, it may swell and produce a nodule which may be felt if the blood vessel is close to the skin surface. If blood flow is reduced or stopped, the tissues which receive blood from that vessel begin to die. The first cause is rare.