Henoch-Schönlein purpura pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Pathophysiology
HSP can develop after infections with streptococci (β-haemolytic, Lancefield group A), hepatitis B, herpes simplex virus, parvovirus B19, Coxsackievirus, adenovirus, Helicobacter pylori, measles, mumps, rubella, mycoplasma and numerous others. Drugs linked to HSP, usually as an idiosyncratic reaction, include vancomycin, ranitidine, streptokinase, cefuroxime, diclofenac, enalapril and captopril. Several diseases have been reported to be associated with HSP, often without a causative link. Only in about 35% of cases can HSP be traced to any of these causes.
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. It is suspected that abnormalities in the IgA1 molecule may provide an explanation for its abnormal behaviour in both HSP and the related condition IgA nephropathy. One of the characteristics of IgA1 (and IgD) is the presence of an 18 amino acid-long hinge region between complement-fixating region 1 and 2. Of the amino acids, half is proline, while the other ones are mainly serine and threonine. The majority of the serines and the threonines have elaborate sugar chains, connected through oxygen atoms (O-glycosylation). This process is thought to stabilise the IgA molecule and make it less prone to proteolysis. The first sugar is always N-acetyl-galactosamine (GalNAc), followed by other galactoses and sialic acid. In HSP and IgAN, it appears that these sugar chains are deficient. The exact reason for these abnormalities are not known.