HSP is a systemic vasculitis (inflammation of blood vessels) and is characterized by deposition of immune complexes containing the antibody immunoglobulin A (IgA); the exact cause for this phenomenon is unknown.

[5] Forty percent have evidence of kidney involvement, mainly in the form of hematuria (blood in the urine), but only a quarter will have this in sufficient quantities to be noticeable without laboratory tests.

[6] Problems in other organs, such as the central nervous system (brain and spinal cord) and lungs may occur, but is much less common than in the skin, bowel and kidneys.

While abnormalities on urinalysis may continue for a long time, only 1% of all HSP patients develop chronic kidney disease.

[8][9] Henoch–Schönlein purpura is a small-vessel vasculitis in which complexes of immunoglobulin A (IgA) and complement component 3 (C3) are deposited on arterioles, capillaries, and venules (hence it is a type III hypersensitivity reaction).

[10] This leads to the production of inflammatory mediators, including vascular prostaglandins like prostacyclin, which may play a key role in the development of IgAV and its organ-specific clinical manifestations.

[12] HSP can develop after infections with streptococci (β-haemolytic, Lancefield group A), hepatitis B, herpes simplex virus, parvovirus B19, Coxsackievirus, adenovirus, Helicobacter pylori,[8] measles, mumps, rubella, Mycoplasma and numerous others.

[12] Drugs linked to HSP, usually as an idiosyncratic reaction, include the antibiotics vancomycin and cefuroxime, ACE inhibitors enalapril and captopril, anti-inflammatory agent diclofenac, as well as ranitidine and streptokinase.

[12] The exact cause of HSP is unknown, but most of its features are due to the deposition of abnormal antibodies in the wall of blood vessels, leading to vasculitis.

These antibodies are of the subclass IgA1 in polymers; it is uncertain whether the main cause is overproduction (in the digestive tract or the bone marrow) or decreased removal of abnormal IgA from the circulation.

[8][12] Multiple standards exist for defining Henoch–Schönlein purpura, including the 1990 American College of Rheumatology (ACR) classification[16][17] and the 1994 Chapel Hill Consensus Conference (CHCC).

[19] More recent classifications, the 2006 European League Against Rheumatism (EULAR) and Pediatric Rheumatology Society (PReS) classification, include palpable purpura as a mandatory criterion, together with at least one of the following findings: diffuse abdominal pain, predominant IgA deposition (confirmed on skin biopsy), acute arthritis in any joint, and renal involvement (as evidenced by the presence of blood and/or protein in the urine).

[20] Henoch–Schönlein purpura may present with an atypical manifestation, which can be confused with papular urticaria, systemic lupus erythematosus, meningococcemia, dermatitis herpetiformis, and acute hemorrhagic edema of infancy.

[23] A systematic review did not find any evidence that steroid treatment (prednisone) is effective at decreasing the likelihood of developing long-term kidney disease.

[12] There is no good evidence that treating children who have HSP with antiplatelet agent prevents persistent kidney disease.

[26] Overall prognosis is good in most patients, with one study showing recovery occurring in 94% and 89% of children and adults, respectively (some having needed treatment).

Proteinuria, hypertension at presentation, and pathology features (crescentic changes, interstitial fibrosis and tubular atrophy) predicted progression.

The number of crescentic glomeruli is an important prognostic factor in determining whether the patient will develop chronic renal disease.

Johann Schönlein described the condition as an entity in 1837; Eduard Heinrich Henoch in 1868 reported the first case of a patient with colic, bloody diarrhea, painful joints, and a rash.

[35] In 2012, the International Chapel Hill Consensus Conference Nomenclature of Vasculitides renamed HSP IgA vasculitis.