MPO and infection: Certain infections, drugs, or airborne particles can trigger AAV by inducing an initially normal immune response, in which C5a–C5aR1 binding and pro-inflammatory cytokines from macrophages prime neutrophils to express bacteria-degrading proteins, such as myeloperoxidase (MPO) or proteinase 3 (PR3), and form neutrophil extracellular traps (NETs).