[LYMPH NODE, NORMAL HISTOLOGY]. Lymph nodes are small lymphoid organs distributed throughout the body at specific locations and connected to other nearby nodes by lymphatic vessels carrying lymph fluid. A node consists of two main regions cortex (long left arrow) and medullar (arrowhead). The blood vessels and efferent lymphatic channel pass through hilum (bottom arrow). The primary function of lymph node is to respond to non-self antigens and generate antigen-specific soluble antibodies from mature plasma cells.
[LYMPH NODE, NORMAL HISTOLOGY]. The lymph node is covered with a continuous collagenous capsule (arrowhead) which is only incomplete at the hilum where a single nodal artery enters the node and a single vein and a single efferent lymphatic vessel exit the node (arrow). The hilar vessels run deep within the node somewhere in the center where influx of medullary cells and drainage of lymph take place.
[LYMPH NODE, NORMAL HISTOLOGY]. In contrast to a single efferent lymphatic channel multiple incoming afferent lymphatic channels enter the node at discrete intervals (arrows) in the capsule. These afferent lymphatic channels drain lymph fluid content inside the node for recognition and response to any non-self antigens. The lymph drains from afferent lymphatic channels to sub-capsular sinuses to cortex and from cortex to medullary sinuses and finally into the efferent lymphatic channel. The lymph fluid essentially consists of plasma with soluble antigens and lymphocytes.
[LYMPH NODE, NORMAL HISTOLOGY]. The node shows two main architectural and functional zones: the outer region just underneath the capsule and sub-capsular sinuses is cortex (arrowhead) and the inner zone contiguous with the hilar vessels is medulla (arrow). The cortex contains lymphoid follicles which are aggregates primarily of B-lymphocytes whereas the medulla contains medullary sinuses (arrow), plasma cells, memory B-cells, some T-cells, and histiocytes.
[LYMPH NODE, NORMAL HISTOLOGY]. The medullary sinuses drain lymph fluid containing T-cells, few B-cells, and numerous macrophages. These cells travel through complex network of lymphatic channels and finally end up either at other lymphoid organs or through blood at sites of inflammation. Unlike in reactive lymph nodes the sinuses in nodes involved by most lymphomas are obliterated.
[LYMPH NODE, NORMAL HISTOLOGY]. Immunostaining for the B-cell marker CD20 shows numerous lymphoid follicles to be situated in the outer cortex (top arrow). The paracortical zone (arrowhead) which is primarily composed of T-cells is devoid of any significant number of CD20+ B-cells. Note also the medullary region in the center that shows a population of post-germinal center memory B-cells.
[LYMPH NODE, NORMAL HISTOLOGY]. Immunostaining for the T-cell marker CD3 shows numerous T-cells in the paracortical zone (arrowhead) and lack of any significant number of CD3+ T-cells in lymphoid follicles (top arrow). The medullary region in the center is devoid of any significant number of T-cells.
[LYMPH NODE, NORMAL HISTOLOGY]. The sub-capsular sinus is composed of fenestrated structure lined by CD31+ endothelial cells. The sub-capsular sinus contains predominantly CD4+ T-cells with bound peptide-MHC II complexes which drain lymph through the cortex where antigenic stimulation causes B-cell activation in primary follicles and formation of germinal centers in secondary follicles.
[LYMPH NODE, NORMAL HISTOLOGY]. A primary follicle is a round to oval structure composed of unstimulated naïve B-cells without a germinal center.
[LYMPH NODE, NORMAL HISTOLOGY]. With antigenic stimulation the primary follicles develop germinal centers where lymphocytes become activated and differentiate eventually to immunoglobulin –secreting plasma cells. This image shows secondary lymphoid follicles with germinal centers (arrow) and a thin interfollicular zone (arrowhead).
[LYMPH NODE, NORMAL HISTOLOGY]. This image shows two secondary B-lymphoid follicles and intervening paracortical zone composed of T-cells. The secondary follicles are composed of an outer mantle zone (#1) and an inner germinal center (#2). The paracortical zone is composed almost entirely of small CD3+ T-cells. The mantle zone B-cells are composed of naïve B-cells and represent outer extension of primary follicular B-cells.
[LYMPH NODE, NORMAL HISTOLOGY]. A secondary lymphoid follicle shows an outer mantle zone (arrowhead) and an inner round to oval germinal center. The germinal center is a region of B-cell activation by antigen-dependent CD4+ T-cells. Two morphologically distinct zones can be identified in the germinal center, a light zone composed primarily of centrocytes (arrow with tail) and a dark zone composed of centroblasts (top arrow). The centroblasts are highly proliferative lymphoid cells which may undergo somatic hypermutations. Neoplastic lymphoid follicles do NOT show polarity (light and dark zones).
[LYMPH NODE, NORMAL HISTOLOGY]. A Ki-67 proliferation stain shows highly proliferative centroblasts (on the right) than slightly less proliferative centrocytes (on the left). In neoplastic follicles of follicular lymphoma no such zoning is identified.
[LYMPH NODE, NORMAL HISTOLOGY]. Immunohistochemical staining for the B-cell marker CD20 shows a normal lymphoid follicle in the cortex. Note the staining in mantle zone cells and germinal center. The paracortex which is largely composed of T-cells is devoid of staining for CD20.
[LYMPH NODE, NORMAL HISTOLOGY]. Staining for the B-cell marker shows a lymphoid follicle with a mantle zone (top arrow with tail). Note that the expression of CD79a is stronger in mantle zone cells because of higher number of CD79a molecules than germinal center B-cells (bottom long arrow) which show fewer CD79a molecules. Note the presence of CD79a+ plasma cells in the light zone of germinal center (arrowhead). The plasma cells move out of the germinal center into the medulla.
[LYMPH NODE, NORMAL HISTOLOGY]. An immunostain for CD3 highlights the paracortical region which is almost entirely composed of small T-cells. Note also the sparse presence of CD3+ T-cells in the mantle zone (#1) and germinal center (#2). These helper T-cells are responsible for antigen-dependent activation of B-cells in the follicle. All T-cells show expression of bcl-2 and thus part of the bcl2 positivity in lymphoid follicles is due to T-cells.
[LYMPH NODE, NORMAL HISTOLOGY]. Immunostaining for CD4 shows that almost all T-cells in the paracortex are of CD4 T-cell helper type. Furthermore, almost all T-cells in the lymphoid follicles are also of CD4 phenotype. Note that more cells appear to be present in the germinal center (#2) with staining for CD4 than with staining for CD3. This is because CD4 receptors are also expressed on macrophages present inside germinal center.
[LYMPH NODE, NORMAL HISTOLOGY]. A small population of CD8+ T-cells is present in the paracortex and in the lymphoid follicle but the number of CD8+ T-cells is much smaller than CD4+ T-cells. The ratio of CD4+ to CD8+ T-cells in the node can range from 2:1 to 10:1 depending upon the type of nodal reactivity to a variety of antigens.
[LYMPH NODE, NORMAL HISTOLOGY]. This image shows a portion of germinal center (arrow) and on overlying mantle zone (arrowhead). Note that the mantle zone B-cells are small in contrast to activated germinal center B-cells. Generally no sharply define or thick mantle zone is seen around neoplastic lymphoid follicles.
[LYMPH NODE, NORMAL HISTOLOGY]. This image shows an inconsistent marginal zone that is sometimes seen outside of mantle zones in some reactive lymph nodes. The marginal zone B-cells contain more cytoplasm than mantle zone B-cells and show centrocytes-like morphology with slightly irregular nuclei (arrow).
[LYMPH NODE, NORMAL HISTOLOGY]. The region of germinal center shows both small and larger cleaved cells (arrowhead) and large non-cleaved cells or centroblasts (arrows). A very large centroblast is shown with a thick arrow. The curved arrow identifies mitosis. Note the presence of multiple mitoses in the germinal center.
[LYMPH NODE, NORMAL HISTOLOGY]. The germinal center is an area of high cell proliferation and active apoptosis. Numerous macrophages are present in a germinal center to engulf and clear dying cell debris. These macrophages are sometimes called “tingible-body” macrophages because of the appearance of debris within their cytoplasm.
[LYMPH NODE, NORMAL HISTOLOGY]. The region of medulla shows numerous plasma cells (arrows). Plasma cells are formed inside germinal centers but move out into the medulla to secrete their soluble immunoglobulins into the efferent circulation.
[LYMPH NODE, NORMAL HISTOLOGY]. All normal germinal center B-cells show expression of CD10 (arrow) but mantle zone B-cells are negative for CD10 expression (arrowhead). Most but not all follicular lymphomas and Burkitt lymphoma also show expression of CD10.
[LYMPH NODE, NORMAL HISTOLOGY]. All germinal center B-cells, whether benign or neoplastic, show nuclear expression of bcl-6 protein (center). Non-germinal center B-cells (arrowhead) and T-cells do not show expression of bcl-6 protein. The expression of bcl-6 only indicates the presence of germinal center follicular cells. Presence of bcl-6 positive cells outside of germinal center suggestive extrafollicular extension of neoplastic follicular cells since bcl-6+ cells are only seen inside germinal centers of benign follicles.
[LYMPH NODE, NORMAL HISTOLOGY]. A reactive lymphoid follicle shows bcl-2 expression by mantle zone B-cells (arrowhead) and all T-cells inside (long thin arrow) or outside of germinal centers.
[LYMPH NODE, NORMAL HISTOLOGY]. Staining for CD21 antigen shows a dense meshwork of follicular dendritic cells which are responsible for antigen presentation.
[NORMAL LYMPH NODE HISTOLOGY]. The follicular dendritic cells (lower arrowhead) also express low-affinity IgE receptor CD23. This expression, however, is not unique to follicular dendritic cells since CD23 expression is also seen on a subset of mantle zone B-cells (arrow). CD23 is used to identify follicular dendritic cell neoplasms in addition to other markers including CD21.
[LYMPH NODE, NORMAL HISTOLOGY]. Sparse scattered macrophages (arrow with tail) are present inside germinal center and are responsible for removing debris of apoptotic cells. Note absence of any macrophages in the mantle zone (arrowhead).
[LYMPH NODE, NORMAL HISTOLOGY]. A small number of CD57+ T-cells and NK-cells are identified inside lymphoid follicle germinal center.
[LYMPH NODE, NORMAL HISTOLOGY]. Interdigitating dendritic cells (IDC) are non-lymphoid, non-phagocytic, antigen-presenting cells situated in the paracortex among follicles. These cells are similar to follicular dendritic cells (FDC) except that they present antigens to T-cells rather than B-cells and thereby help in T-cell activation. The antigens are derived from lymph fluid percolating through the paracortext.
[LYMPH NODE, NORMAL HISTOLOGY]. Note the long cell processes of interdigitating cells involved in network of interactions with other cells. These interdigitating networks allow cells to pass antigens to neighboring IDC cells for an amplified immune response. Note the intimate contact of interdigitating cells with surrounding lymphocytes.
[LYMPH NODE, NORMAL HISTOLOGY]. A single interdigitating dendritic cell is shown with fascin immunohistochemistry. Note the long cytoplasmic processes responsibe for antigen presentation to T-cells.
[LYMPH NODE, NORMAL HISTOLOGY]. The paracortex and medulla contain blood vessels with high endothelial cells.
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