Fibromyalgia syndrome (FMS) is characterized by elevated levels of immunoglobulin G (IgG), altered bowel habits, and increased pain sensitivity, suggesting immune dysregulation, but the exact mechanism remains unclear. Here, we found that FMS-IgG binds to mast cells in a MRGPRX2/b2-dependent manner, leading to mast cell recruitment and IL-6 secretion. Transferring serum-IgG from FMS patients to mice induced FMS-like symptoms and increased skin mast cells, indicating that FMS-IgG acts through mast cell activation. The ablation of mice Mrgprb2 mast cells or deleting Mrgprb2 receptors prevented IgG-induced heightened sensitivity to mechanical and cold stimuli. Stimulating human LAD2 cells with FMS IgG elicited MRGPRX2-dependent IL-6 production. Consistent with mice findings, mast cell density and tryptase levels increased in human FMS skin samples compared to healthy controls. Taken together our results suggests that FMS IgG mediates hypersensitivity via activation of mast cells bearing the MRGPRX2 receptor and that these cells are a potential therapeutic target. ### Competing Interest Statement X.D. is the scientific founder of and consultant for Escient Pharmaceuticals, a pharmaceutical company developing drugs targeting Mrgprs. X.D. collaborates with GlaxoSmithKline (GSK) on Mrgpr-related projects unrelated to this manuscript. Other authors declare no competing interests. Howard Hughes Medical Institute, https://ror.org/006w34k90 Pain Relief Foundation, https://ror.org/0017mh436 Versus Arthritis, https://ror.org/02jkpm469, 22471

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