PRTN3 and rheumatoid arthritis: Evidence of continuous activation of the response can be derived from the fact that IgM ACPA are present in sera of patients with both early undifferentiated arthritis and established RA.101, 102 Moreover, ACPA IgM can (re‐)appear throughout the course of the disease.101 Together with the signs of abundant SHM in citrulline‐reactive B cells, these observations indicate that the activation of ACPA/AMPA B cells is an ongoing process.