Henoch-Schönlein purpura (lgA vasculitis)

Pathogenesis

• Deposition of lgA immune complexes in small vessels activate complement
• Neutrophilic inflammation & vascular damage
• Often follows an upper respiratory infection

Clinical manifestation

• Palpable purpura/petechiae on the lower extremities
• Arthritis/arthralgia
• Abdominal pain, gastrointestinal bleeding, intussusception
• Renal disease (hematuria ± proteinuria)

Diagnosis

• Usually clinical
• Skin biopsy: leukocytoclastic vasculitis, lgA deposition in vessel walls.

lgA nephropathy:

• Mesangial deposition of lgA immune complexes triggers mesangial cell proliferation and extracellular matrix deposition, primarily via enhanced expression of platelet-derived growth factor and transforming growth factor beta.
• lgA immune complexes also trigger complement activation via the alternative and lectin pathways; hydrolyzed complement proteins (eg, c5a) act as chemokines that attract neutrophils, which produce oxidants and release proteases (eg, elastase) that are damaging to glomerular capillaries.
• Macrophages may also be recruited to the glomerulus, where they cause further damage to glomerular capillaries, promote fibrin deposition, and contribute to crescent formation in severe cases.