Henoch-Schonlein is a type of hypersensitivity vasculitis and inflammatory response within the blood vessel. It is caused by an abnormal response of the immune system. The exact cause for this disorder is unknown.

The syndrome is usually seen in children, but people of any age may be affected. It is more common in boys than in girls. Many people with Henoch-Schonlein purpura had an upper respiratory illness in the previous weeks.