Leukotriene B4 receptor 1 (BLT1) triggers the migration of granulocytes and activated T cells; however, its role in B-cell function remains unclear. Here we report that BLT1 is required to induce the production of antigen-specific IgA against oral vaccine through mediating innate immune signals from commensal bacteria. B cells acquire BLT1 expression during their differentiation to IgA+ B cells and plasma cells in Peyer's patches and the small intestinal lamina propria, respectively. BLT1 KO mice exhibited impaired production of antigen-specific fecal IgA to oral vaccine despite normal IgG responses to systemically immunized antigen. Expression of MyD88 was decreased in BLT1 KO gut B cells and consequently led to diminished proliferation of commensal bacteria-dependent plasma cells. These results indicate that BLT1 enhances the proliferation of commensal bacteria-dependent IgA+ plasma cells through the induction of MyD88 and thereby plays a key role in the production of antigen-specific intestinal IgA.