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Kawasaki disease (KD) is an acute, systemic vasculitis syndrome that occurs in children. The clinical symptoms and epidemiologic features of KD strongly suggest that KD is triggered by unidentified infectious agents in genetically predisposed patients. In addition, a number of studies have described the role of B cells in the development of KD. To obtain a mechanistic insight into the humoral immune response of B-lineage cells in KD patients, we examined peripheral blood antibody secreting cells (ASCs) and inhibitory immunoreceptors, immunoglobulin-like transcript (ILT)/leukocyte immunoglobulin-like receptor (LILR), on each B cell subpopulation.Eighteen Japanese KD patients and thirteen healthy control subjects were recruited for this study. Their peripheral blood mononuclear cells were examined by flow cytometry for the number of CD19+ B cells, the size of each B cell subset and the expression of the inhibitory isoforms of ILT/LILR on the B cell subset.The frequency of CD19+CD27high ASCs was significantly increased in the acute phase of KD and reduced after high-dose intravenous immunoglobulin (IVIG) treatment. Interestingly, while ILT2/LILRB1 expression was ubiquitously observed on every B cell/ASCs subset and the level was not significantly different after IVIG, ILT3/LILRB4 (B4) was uniquely expressed on only ASCs, and its expression was significantly decreased after IVIG.In the acute phase of KD, the frequency of ASCs is high with augmented B4 expression, whereas it is lower with decreased B4 expression after IVIG. Further studies of B4 expression on ASCs in autoimmune and infectious diseases will be needed to confirm the significance of our findings.