Histologic and immunohistologic images of normal germinal centers.

(A) Hematoxylin and eosin-stained sections of a normal tonsil shows a germinal center with polarized dark and light zones. The dark zone shows rapidly proliferating cells as evidenced by increased mitotic activity and tingible body macrophages. The light zone is pale stained and shows a low proliferative activity; (B) An immunohistologic stain for Ki-67 (MIB1) highlights increased proliferation in one pole of the secondary follicle corresponding to the dark zone; (C) An immunohistologic stain for CD21 highlights the follicular dendritic cell meshwork within the germinal center; (D) An immunohistologic stain for IgM highlights immune complexes captured on follicular dendritic cell processes that are involved in antigen presentation to germinal center B-cells.