IgA vasculitis (Henoch-Schönlein purpura)

It is a small-vessel vasculitis driven by misdirected IgA

IgA vasculitis follows a triggering event, usually an upper respiratory tract infection, in which abnormally glycosylated IgA1 forms immune complexes that deposit in the walls of small vessels (capillaries, venules) throughout the body. Complement activation and inflammation at these deposition sites damage the vessel wall, causing leakage of blood into surrounding tissue and organ dysfunction wherever a vessel bed is affected. Because the trigger is systemic but IgA deposition is patchy and unpredictable in location, the disease can affect any organ with a small-vessel supply, which is why the clinical picture looks like a scattered multisystem illness rather than a single organ disease.