Rheumatoid arthritis (RA) is an autoimmune disease that induces chronic arthritis by acting on autoantigens mainly expressed in joints. ACPA, an autoantibody of RA that reacts with citrullinated proteins, is not only used as a diagnostic marker but has also been reported to be involved in the pathogenesis of RA. On the other hand, IgG has a biantennary N-glycan attached to the 297th asparagine in the Fc region. The branched glycans are sequentially added with GlcNAc, galactose (Gal), and sialic acid (Sia) to mannose (Man), but the extension level is uncertain, resulting in diversity in the glycan structure (Fig. 63.1a). In 1985, it was reported that the amount of serum IgG lacking sialic acid and galactose increases in RA patients [1] (Fig. 63.1b). Since then, research on autoantibodies and glycans in RA has progressed, and it has been reported that the lack of terminal sialic acid in IgG enhances the inflammatory action of RA, and when terminal sialic acid is added to IgG, it induces anti-inflammatory action [2].