Detection of large B cells of dendritic morphology in interfollicular regions of lymph node and tonsil, and in thymus. (A) Lymph node: Sections of 2 lymph nodes immunostained for CD20. The low-power views (× 10) on the left show scattered CD20⁺ B cells in the T-cell–rich areas that lie between and around lymphoid follicles (Foll). The boxed areas in these low-power views are seen at higher power (× 40) to the right.Arrows indicate processes extending from interfollicular large B cells. (B) Tonsil: Confocal images from a section stained for CD20 (red) and OCT-2 (green). In the upper half of the picture, 2 adjacent cells are illustrated, seen as a single optical section (left) or as multiple sections flattened into a single image (right). A flattened image of a second cell is seen in the lower part of the image. Magnification for all images, × 60. Note the striking dendritic morphology of the cells visualized by this technique. (C) Hyper-IgM syndrome lymph node: CD20 staining (magnification, × 10) shows prominent primary follicles (Foll) but only one interfollicular large B cell (an example is indicated by an arrow and shown at higher magnification [× 40]). (D) Thymus: CD20 staining (magnification, × 10) of large B cells in the thymus, lying principally in the medulla around a Hassall corpuscle (HC). In the inset, high magnification (× 40) of these cells shows morphology similar to cells seen in lymph node. (E) Thymus: Double labeling for CD20 (red) and CD3 (green) (magnification, × 20) shows the differing numbers of cells in the medulla and the cortex (left panel), and two B cells in the cortex, with processes penetrating between surrounding T cells (right panel and inset). Inset magnification, × 40.