PRTN3 and rheumatoid arthritis: Recognition of defined citrullinated antigens by IgM ACPA was restricted to samples that also displayed recognition by IgG ACPA, but the IgM ACPA response showed a more restricted antigen recognition profile than IgG ACPA.[40] Moreover, no IgM CCP2 responses were observed in IgG CCP2 negative RA patients[41] or in healthy controls[42], suggesting that IgM CCP2 responses are rather specific for ACPA positive RA patients.