The normal or benign lymph node is composed of a collection of follicles and interfollicular areas surrounded by sinuses (spaces mostly filled with histiocytes), vessels, and sometimes fat. Lymph flows from the subcapsular sinuses, through the medullary sinuses in the lymph node, and out the hilum. The follicles represent areas of maturing B cells (CD20+), whereas the interfollicular areas are mostly mature T cells (CD3+).
Follicles begin as primary follicles, or aggregates of antigen-naïve B cells. As they mature into secondary follicles they acquire germinal centers, which are visible as targetoid nodules within the follicles (Figure 21.1). The dark outer rim of the follicle is the displaced remains of the primary follicle and is called the mantle zone, still composed of antigen-naïve B cells. Once exposed to antigen, the B cells move to the germinal center and become centroblasts, large cells with primitive-looking nuclei. From there they either mature into centrocytes or die through apoptosis. Finally, B cells leave the germinal center genetically altered to circulate as memory B cells, which may ultimately differentiate into plasma cells if they meet their antigen.
Other normal germinal center components include the supporting follicular dendritic cells and tingible body macrophages, which clean up the apoptotic debris. These appear as relatively clear cells within the germinal center, with visible “dust specks” in the cytoplasm (Figure 21.2). Germinal centers may be found in any hotbed of lymphocyte activity outside the lymph nodes, but the morphology and staining pattern are preserved.
The paracortex, or interfollicular area, may occupy most of the lymph node in some cases. This absence of obvious follicles is not necessarily a reason for concern. The benign paracortex should have a mottled appearance due to the scattered pale histiocytes among the T cells.
KeywordsFollicular Lymphoma Germinal Center Mantle Cell Lymphoma Anaplastic Large Cell Lymphoma Marginal Zone Lymphoma
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