IRTA1 expression in Peyer patches. (A) Expression of IRTA1 is topographically restricted to the marginal zone (MZ) including isolated or small clusters of intraepithelial B cells (arrows) in the dome epithelium; cells in the germinal center (GC) and mantle (M) of B-cell follicles are IRTA1^-. (B-C) Other areas of Peyer patches showing, at higher magnification, that IRTA1 is selectively expressed by marginal zone (MZ) B cells including intraepithelial B cells. The arrow in panel B and the arrowheads in panel C point to IRTA1⁺ intraepithelial B cells. APAAP technique with mAb M-IRTA1; hematoxylin counterstain. Original magnifications × 100 (A); × 1000 (B-C).