Double-stained GC of normal tonsil (paraffin sections).

(A) Double staining for MUM1 (brown) and CD23 (red) shows that GC MUM1⁺ cells (arrowhead) are located in the light zone in close association with CD23⁺ follicular dendritic reticulum cells (arrow). Asterisk indicates negative centroblasts in the dark zone (× 250; hematoxylin counterstain). (B) The intimate contact of brown MUM1⁺ cells (arrowhead) with red CD23⁺follicular dendritic cells (arrow) is shown at higher magnification (× 1000). Double arrowheads indicate a MUM1⁻/CD23⁻ GC cell. (C) Expression of MUM1 (red) and Bcl-6 (green) are mutually exclusive within the GC of tonsil (double immunofluorescence; × 800). (D) Expression for brown nuclear Bcl-6 (arrowhead) and blue nuclear MUM1 (arrow) in GC B-cells appears to be mutually exclusive (× 1000; no counterstain). (E) Expression for brown nuclear MUM1 protein (arrowhead) and blue Ki67 proliferation antigen (arrow) appears to be mutually exclusive, with the exception of rare cells (double arrowheads) that double stain for the 2 antigens (× 1000; no counterstain). (F) The MUM1 protein (brown) and the CD68 antigen (blue) are clearly expressed in different cell types. The arrow points to a CD68⁺ tingible body macrophage, whereas the arrowhead indicates a MUM1⁺ GC cell (× 1000; no counterstain). (A-F) Biotin-avidin peroxidase/APAAP.