Immunostaining of interfollicular large B cells for different markers. (A) Left panel: Some interfollicular large B cells express the proliferation-associated marker MIB-1 (Ki-67) (double immunofluorescence staining for CD20 in red and MIB-1 [Ki-67] in green). Note the numerous proliferating cells in a lymphoid follicle and also scattered interfollicular MIB-1⁺CD20^– cells (magnification, × 20). In the inset, 3 different interfollicular large B cells, 2 of which express MIB-1, can be seen (magnification, × 40). Center panel: Interfollicular large B cells lack p27 (inset; magnification, × 40), as shown by double immunoenzymatic staining for this molecule (brown) and CD20 (blue; magnification, × 20). In the germinal center of a follicle most B cells are p27^–. Right panel: Double immunofluorescence labeling (CD20 red; BCL-2 green) shows that interfollicular large B cells lack BCL-2. Magnification, × 20 for the main panel and × 40 for the inset. (B) Left panel: Double immunofluorescence staining reveals that interfollicular large B cells are CD20⁺ (red) but CD30^– (green). The inset shows 4 examples of CD20⁺ interfollicular large B cells lying near CD30⁺ cells. Magnification is × 40 for all images. Center and right panels: No labeling of interfollicular large B cells is seen in sections stained for the complement receptors CD21 (center panel) and CD35 (right panel) (immunoperoxidase staining). Magnification is × 20 for both images. (C) Double immunofluorescence labeling for CD20 (green) and CD208 (DC-LAMP) (red) shows that interfollicular large B cells do not express the latter dendritic cell–associated marker, which labels a clearly distinct population of cells (magnification, × 20). In the high-power view (× 40) on the right, a typical interfollicular large B cell is indicated by an arrow. The inset shows a CD208^– interfollicular large B cell at higher magnification. (D) Left panel: Double immunofluorescence staining shows no tendency of CD8⁺ T cells (green) to associate with interfollicular large B cells (red) (magnification, × 20). Right panel: High-power views (× 40) show a single example of an interfollicular large B cell (asterisk) in intimate contact with a CD8⁺ cell. Note the extensive dendritic processes extending from 2 other interfollicular large B cells (arrowed). The inset shows a CD8^– interfollicular large B cell at higher magnification.