IRTA1 expression in paraffin sections from normal tonsil immunostained with the mAb M-IRTA1. (A) IRTA1⁺ cells are selectively distributed within the tonsil epithelium (e) and at the border of mantle zones. ^* indicates a IRTA1^- germinal center. (B) IRTA1⁺ cells often show a villous cell surface (arrow). (Inset) Blue indicates staining for IRTA1; brown, the proliferation antigen MIB1. Most IRTA1⁺ intraepithelial B cells are not proliferating. The arrow points to a large IRTA1⁺/MIB1⁺ cell. (C) IRTA1⁺ B cells within the epithelium (e) often form a rim (arrow) around clusters of subepithelial plasma cells (arrowhead) arranged around small vessels. Inset shows higher magnification of an IRTA⁺ cell. (D) Double staining for IRTA1 (blue) and MUM1/IRF4 (brown) shows a rim of IRTA1⁺/MUM1^- negative intraepithelial B cells surrounding a cluster of IRTA1^-/MUM1⁺ subepithelial plasma cells (arrow). The APAAP technique was used in panels A-C; double immunoperoxidase/APAAP staining, panels D and B inset. Original magnifications × 100 (A); × 1000 (B, larger panel and inset); × 400 (C-D).