FFAR2 and rheumatoid arthritis: AITD: autoimmune thyroid disease; SLE: systemic lupus erythematosus; P1D: primary immunodeficiency disease; MS: multiple sclerosis: RA: rheumatoid arthritis; AID: autoimmune diseases; SCFAs: short-chain fatty acids; TLR7: Toll-like receptor 7; GPR43: G-protein-coupled receptor 43; HDAC: histone deacetylase