MRGPRX2 and rheumatoid arthritis: Futhermore, it has later appeared that PMX53 is an agonist of Mas-related gene X2 (MrgX2) and via this receptor can induce mast cell degranulation [60, 61].The anti-C5 antibody, eculizumab, has also been reported to lack efficacy in patients with RA [1], although details has not been published.