Henoch–Schönlein purpura (HSP, also known as anaphylactoid purpura, purpura rheumatica, and Schönlein–Henoch purpura) is a disease of the skin and other organs that most commonly affects children. In the skin, the disease causes palpable purpura (small hemorrhages); often with joint and abdominal pain. With kidney involvement, there may be a loss of small amounts of blood and protein in the urine, but this usually goes unnoticed; in a small proportion of cases, the kidney involvement proceeds to chronic kidney disease. HSP is often preceded by an infection, such as pharyngitis.
HSP is a systemic vasculitis (inflammation of blood vessels) and is characterized by deposition of immune complexes containing the antibody IgA; the exact cause for this phenomenon is presently unknown. It usually resolves within several weeks and requires no treatment apart from symptom control, but may relapse in a third of the cases and cause irreversible kidney damage in about one in a hundred cases.