Henoch-Schonlein purpuraHenoch - Schonlein purpura is vasculitis of small vessels (capillaries, venule or arterioles) and characterized by deposition of IgA in the wall of involved vessels. H.S. purpura is characterized by a tetrad of purpura, arthritis, glomerulonephritis, and abdominal pain. All changes resulting from the deposition of circulating immune complexes, containing IgA and complement on the endothelium of the blood vessels throughout the body and within the glomerular mesangial regions.