PRTN3 and rheumatoid arthritis: Circulating IgA class ACPA arise prior to symptom onset in a substantial proportion of patients (30%–40%)18 and are prognostic for RA onset both before and after symptom onset.18 19 IgA responses are often ‘associated by default’ to mucosa even though most of the IgA found in circulation originates from long-lived plasma cells in the bone marrow.