AID is expressed in the dark zone and the outer zone in GC. (A) AID (purple) and CD23 (red) are mutually exclusive in the GC. Note weakly CD23⁺ mantle zone. Original magnification ×4. (B) CD3⁺ T cells (red) are abundant in the AID⁺ outer zone at the right edge of the GC. Original magnification ×4. (C-D) The 2 white rectangles shown in panel A are magnified. In panel C, the basal light zone is empty, flanked by AID⁺ dark zone on top left, containing a single nAID⁺ cell (arrow) and the CD23⁺ apical light zone in the bottom right, in which a weakly AID⁺ nucleus contact CD23⁺ FDC (arrow). In panel D, the outer zone is magnified, containing numerous nAID⁺ cells (arrows). Original magnification ×40. Note the CD23⁺ mantle zone cells in the lower-right corner. (E) High-power detail of the area boxed in panel G, showing IgD strongly positive mantle zone cells on the right (purple), and double BCL6 (red) and IgD weakly positive cells of the outer zone, in the center. (F-G) Low-power images of serial sections stained for IgD (purple) and Ki-67 (F) or BCL6 (G). Note the double-positive cells under the IgD⁺ mantle zone. Scale bar: 1 μmat ×40, 4 μmat ×10, and 10 μmat ×4.