B220 immunoreactivity in mesenteric lymph nodes of a 4-month-old transgenic mouse (A) and a control littermate (B). No follicle structures and no B220-immunoreactive cells are observed in the transgenic mouse. The control mouse has follicles with germinal centers (arrows) immunoreactive for B220. (Original magnification × 75; PGC-fixed and paraffin-embedded section; ABC immunohistochemistry; hematoxylin.)