B-cell follicles and their neoplastic transformation. Hematoxylin and eosin–stained spleen sections from (A) a 16-month-old CD137L^−/− mouse diagnosed with early-stage FL showing merging of enlarged GCs; (B) an 11-month-old CD27/CD137L^−/− mouse diagnosed with late-stage FL showing nodular appearance of tumor mass; (C) a 12-month-old WT mouse, in which the GC (G, dotted circle) is small, and the periarteriolar lymphoid sheath region (*) and mantle zone (MZ) are prominently present, and (D) a 10-week-old WT mouse at day 8 after influenza infection. The GC is enlarged, and the periarteriolar lymphoid sheath region is less pronounced. Original magnifications panels A-D ×2.5, insets ×40.