Immunostaining of interfollicular large B cells for pan–B-cell markers and B-cell–associated transcription factors. (A) Staining of pan–B-cell markers. Upper left panel: By the immunoperoxidase technique scattered CD79⁺ (brown) large cells are seen in an interfollicular region (and shown at higher magnification [× 40] in the inset). Right panel: Immunofluorescence staining for CD20 (red) in conjunction with CD79a (green). The upper row illustrates the staining pattern of the 2 markers separately and together in the same area at low magnification (× 10). The 2 cells in the interfollicular area (upper row, arrows), one of which shows only weak expression of CD79a, are seen at higher magnification (× 40) in the middle row. The bottom row shows other interfollicular large B cells expressing CD20 and CD79a to illustrate the intracellular localization of the latter marker (magnification, × 60). Lower left panel: CD75 strongly labels B-cell follicles (magnification, × 20) and interfollicular large B cells (as shown at higher magnification [× 40] in the inset). (B) Staining of B-cell–associated transcription factors. Double immunoenzymatic staining for CD20 (blue) and a transcription factor (brown). In each image, lymphoid follicles, or part of a follicle, are seen, containing cells expressing the transcription factor. The CD20⁺ interfollicular large B cells express 3 B-cell–associated transcription factors (as shown at higher magnifications in the insets), but lack PU-1. Magnification is × 20 for all main images and × 40 for the insets.