H2R is highly expressed in neutrophils. (A) Relative expression levels of Hrh1 and Hrh2 in different cell types of female C57/B6 mice by qRT-PCR (n = 4). (B) The gene expression pattern of histamine receptors in mouse intestinal epithelial cells obtained from the single-cell portal (GEO: GSE92332). (C) The gene expression pattern of histamine receptors in human intestinal epithelial cells obtained from the single-cell portal (SCP259). (D) The relative gene expression of Hdc, histamine receptors, and surface markers in different immune cells obtained from the Gene Skyline database (http://rstats.immgen.org/Skyline /skyline.html). (E) The gene expression pattern of histamine receptors in human immune cells analyzed using the Human Immune Cell Gene Viewer. mDC, myeloid dendritic cell; mRNA, messenger RNA; NK, Natural killer; PBMC, peripheral blood mononuclear cell; pDC, plasmacytoid dendritic cell; TA, transit amplifying; Th, T helper; Treg, regulatory T cells.