MPO and Autoimmunity: Two principal antigens on neutrophils, namely proteinase 3 (PR3) and myeloperoxidase (MPO), provide epitopes for ANCA binding, thus promoting neutrophil activation and neutrophil extracellular traps (NETs) formation (“NETosis”), which consists of the extrusion of lattice-like chromatin fibers harboring cytokines and antimicrobial proteins that contribute to host defense under physiological conditions and promote endothelial damage and vascular inflammation culminating in necrotizing vasculitis in the context of autoimmunity (5–7).