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Allergic asthma is a disease driven by T helper 2 (Th2) cells, eosinophilia, airway hyperresponsiveness (AHR) and IgE-secreting B cells. Asthma is largely controlled by corticosteroids and β2 adregenic receptor agonists that target and relax airway smooth muscle (ASM). Immunoglobulin M (IgM) isotype secreted by naïve B cells is important for class switching but may have other undefined functions. We investigated the role of IgM in a house dust mite (HDM)-induced Th2 allergic asthma model by sensitising wild type (WT) and IgM-deficient (IgM-/-) mice with HDM. We validated our findings using CRISPR and single cell force cytometry in human ASM. We found IgM to be essential in AHR but not Th2 airway inflammation or eosinophilia. RNA sequencing of lung tissue suggested that IgM regulated AHR through modulating brain-specific angiogenesis inhibitor 1-associated protein 2-like protein 1 (Baiap2l1) and erythroid differentiation regulator 1 (Erdr1). Deletion of BAIAP2L1 and ERDR1 reduced human ASM contraction when stimulated with TNF-α. These are unprecedented findings and have implications in future treatment of asthma beyond current therapies.