IRTA1 expression in reactive lymph nodes. (A) Double APAAP staining for IRTA1 (mAb M-IRTA1) and the proliferating antigen MIB1 in a reactive lymph node. IRTA1 strongly labels in red B cells (arrow) at the border of the mantle zone (m) while MIB1 (blue) is mainly expressed by germinal center B cells. A small percentage of double-stained IRTA1⁺/MIB1⁺ cells is present (arrowhead and inset). (B) Double APAAP staining for IRTA1 (mAb M-IRTA1) and BCL6 in a reactive lymph node. IRTA1 strongly labels in red B cells of the subcapsular sinus (arrow), whereas BCL6 (blue) is strongly expressed by germinal center B cells. (C) Monocytoid B cells of toxoplasmic lymphadenitis strongly express IRTA1 (APAAP technique with mAb M-IRTA1). (D-F) IRTA1 expression in human spleen. (D) Spleen stained with polyclonal anti-IRTA1 (H&E stain). The GC is marked by ^*; the marginal zone is bracketed in white. (E) Adjacent section stained for IRTA1 (black) and the B-cell antigen PAX5 (red nuclear staining). (F) Serial sections stained for IgD (black) and PAX5 (red). IRTA1⁺ B cells are distributed in the IgD-dim marginal portion of the marginal zone. Other areas are negative. Original magnifications × 100 (A); × 1000 (A inset); × 200 (B); × 100 (C); × 100 (D-F, top); × 400 (D-F bottom).