COL17A1 and rheumatoid arthritis: Several recent studies have demonstrated that autoreactive IgM can be causative for autoimmune symptoms as anti-BP180 IgM leads to IgM pemphigoid [65], anti-PT IgM antibodies interfere with blood coagulation in severe COVID-19 patients [51,78], or AMPA-IgM as disease driver in RA [73].