MPA affects small vessels and is pauci-immune (ie, immune globulin deposition is not seen on tissue biopsy), similar to granulomatosis with polyangiitis (GPA) and eosinophilic granulomatosis with polyangiitis (EGPA), which differentiates it from immune complex-mediated small-vessel vasculitides (eg, immunoglobulin A-associated vasculitis —formerly known as Henoch-Schönlein purpura) and small-vessel cutaneous vasculitis.