HTR2A and rheumatoid arthritis: One serotonin receptor known as HTR2A is a G-protein-coupled receptor expressed on immune cells including B cells, T cells, macrophages, monocytes, dendritic cells (DCs), natural killer cells (NKs), and eosinophils [10–15], of which gene polymorphisms link to increased susceptibility to RA and other autoimmune diseases [16, 17].