The interaction between neutrophils and activation of alternative complement pathway plays a critical role in the pathogenesis of ANCA-associated vasculitis (AAV). MPO， which can be released from ANCA-stimulated neutrophils， was recently demonstrated to be capable of activating the alternative complement pathway. Here we aimed to investigate the interaction between MPO and factor H (FH)， a key regulator of the alternative pathway， and its effect on the functional activities of FH.