Figure 13-49.

Benign Follicular Hyperplasia, Lymph Node, Dog.

Antigenic stimulation results in a secondary follicle with germinal center formation (G). The centroblasts of the germinal center undergo somatic mutations and isotype class switching, a process during which most centroblasts undergo apoptosis and cell fragments are phagocytized by tingible body macrophages. The cells that have survived the affinity maturation process (centrocytes) leave the germinal center as plasma cell precursors. Some of these cells migrate to the medullary cords, where they mature and excrete antibody into the efferent lymph, whereas others colonize the region surrounding the mantle cell zone to form a marginal zone. In instances of strong antigenic stimulation, the mantle cell cuffs can completely encircle the germinal center (arrows). H&E stain.

(Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.)