ABSTRACT
The lymph nodes act as a scaffolding system and home for lymphocytes and monocytes/histiocytes in the lymphatic system. Lymph nodes (Figure 2.1) are ovoid encapsulated structures composed of cortex (with primary and secondary follicles), paracortex (area between superficial cortex and medulla), and medulla (medullary cords with vessels and sinuses). The capsule and its extension within the lymph node parenchyma (trabeculae), together with a reticular meshwork, form supportive elements of the lymph node. The reticular meshwork is composed of reticular cells, dendritic cells, macrophages, and follicular dendritic cells. Primary and secondary follicles are distributed within the cortex at the periphery of the lymph node. Primary follicles are composed of small and relatively monotonous B cells, which are CD10– and BCL6– and positive for BCL2. Secondary (or reactive) follicles are composed of two zones: central, pale-staining germinal centers and a darker-staining mantle zone composed of mostly small lymphocytes. The mantle zone in the lymph nodes is usually homogenous without an overt marginal zone (Figure 2.2a), typical for follicles in the spleen. Certain reactive conditions in the lymph node may lead to the formation of an easily identifiable marginal zone composed of so-called monocytoid B cells (Figure 2.2b–c). The germinal-center B cells express CD10 and BCL6 and are negative for BCL2. They contain numerous larger lymphocytes with nucleoli (centroblasts), centrocytes (lymphocytes with irregular nuclei), small lymphocytes, tingible body macrophages, and dendritic reticulum cells. Few scattered T cells expressing CD10 and PD1 are also present within reactive follicles [1]. The secondary follicles often show polarization of their architecture (Figures 2.3 and 2.4) with one pole composed of centrocytes (lighter zone) and the other with an increased number of centroblasts and macrophages (darker zone). Polarization helps to differentiate reactive follicles from follicular lymphoma (FL). The polarization is easy to appreciate with Ki-67 (Figure 2.3) or PD1 staining. Supporting the B cells in the follicles are follicular dendritic cells (FDCs), best visualized by staining with CD21 and CD23. Intact (compact) distributions of the FDC meshwork favor a reactive process, whereas an expanded or disrupted meshwork is seen in lymphomas.
