This section will cover an overview of the lymph node structure in relation to its functional use. This includes:
2. Lymph Node Structure
b. Germinal Centers
e. High Endothelial Venules
f. Reticular Meshwork
3.How lymph nodes are connected with the lymphatic system
Lymph nodes are kidney-shaped structures found at regular intervals in the lymphatic system. They are located all over the body, excluding the CNS, and range in size from 1mm to 25mm. Each lymph node is surrounded by a capsule made from a dense layer of connective tissue.
They are a secondary lymphoid organs and their role is to filter lymph and assist in the immune response. Lymphatic vessels pick up antigenic material, inflammatory mediators and antigen presenting cells (APCs) and deliver them to lymph nodes. APCs can interact with naïve lymphocytes and activate an immune response.
The lymph node is divided into 3 areas. The outermost part is the cortex, the innermost is the medulla and the paracortex is in the middle of the two. These are shown in figure 1. B and T cells are located in separate parts of the lymph node. This allows separate areas for the cells to interact with APCs and undergo clonal expansion.
The cortex is a B Cell area containing aggregations of B cells within follicles. In an unstimulated lymph node these follicles are known as primary follicles. Upon stimulation by antigens, they become secondary follicles. Secondary follicles have germinal centers in which activated B cells undergo proliferation and differentiation into plasma cells and memory B cells.
When antigen enters a lymph node it can filter through follicles in the cortex. Here the antigen can be captured and presented to B cells. If the B cell recognizes the antigen, they can take up the antigen and process it. Antigenic peptides are then presented on their class II MHC. B cells can also be partially activated by the antigen and lead to B cell migration to the cortex-paracortex border in the node. Here they encounter CD4 T cells (T helper cells) that have also migrated to the border. Activated B cells express class II MHC on their surface in order to present antigen to the CD4 T cells via their T cell receptor (TCR). If recognition occurs, the CD4 T cells are stimulated to produce cytokines that bind to the B cells. There also needs to be an additional signal of the CD154 ligand on the T cell binding to the CD40 ligand on the B cell. These signals stimulate the B cells to proliferate and form a cluster at the cortex-paracortex border. This interaction is shown in figure 2.
After 4-7 days some of the activated B cells and CD4 T cells migrate to primary follicles, where germinal centers will form. Primary follicles contain follicular dendritic cells (FDCs), a type of APC. FDCs have long processes that are used to contact B cells. They are specialized to capture antigen in the form of immune complexes as well as complexes of antigens, complement and antibodies. The migration of the cells is due to the chemokine B lymphocyte chemoattractant, released by FDCs and other cells in the B cell area, binding to the CXCR5 receptor expressed on B and CD4 T cells.
Within the germinal center, the B cells undergo affinity maturation in which they down regulate their antibody and undergo extensive proliferation. This occurs in the dark zone of the germinal center. The B cells are now known as centroblasts. During proliferation, the cells also undergo somatic mutation in the hypervariable regions of H and L chain genes of the antibody. This alters the hypervariable region of the antibody, which can increase, decrease or have no effect on the antibody’s affinity for the antigen.
The centroblasts then stop proliferating and re-express their membrane antibody. They are now know as centrocytes and are located in the basal light zone of the germinal center. The centrocytes must recognize the antigens on the surface of the FDCs with a strong affinity to receive a survival signal from the FDCs. If they do not, they will die by apoptosis and subsequently be engulfed by macrophages. There is competition between B cells for the antigen on the FDCs so only those cells that bind with high affinity will survive. B cells also undergo a process known as class switching during the centroblast/centrocyte stage. This is a stage in which the heavy chain constant regions can switch. This does not alter the antigen specificity. CD4 T cells and cytokines control this process.
The last stage of B cell differentiation occurs in the apical light zone of the germinal center. Here, they differentiate into either plasma cells,which produce large amounts of antibody, or B memory cells. Plasma cells can either remain in the secondary lymphoid organs or migrate to the bone marrow. B memory cells continue to recirculate through the lymphatic system.
The paracortex is mainly a T Cell area. The lymphocytes migrating from the blood into the lymph nodes also enter here via high endothelial venules. The mechanism of entry is described in more detail later is section e.
Activated dendritic cells (DCs) travel to the paracortex of the lymph node from tissue via lymphatic vessels in response to antigen stimulation. This migration is due to the chemokine Epstein Barr virus-induced receptor ligand (ELC) binding to the chemokine receptor, CCR7, which is upregulated in a stimulated DC. During this migration, DCs process the antigen and express antigen fragments on class II MHC for the CD4 T cells to recognise. Antigens can also enter the lymph node on their own and are then picked up by DCs in the paracortex and presented on class II MHC molecules in the same way as above. The dendritic cell is therefore an APC.
When the DCs migrate to the T-cell areas of the node, they start to secrete ELC, which binds to CCR7 receptors on T-cells, therefore attracting T-cells to DCs in a non-antigen specific manner. CD4 T cells have the integrin LFA-1 on their cell surface that transiently binds to ICAM-1 on DCs. If the T cell does not recognize the antigen it will dissociate from the DC and continue to travel through the lymph node and bind to further DCs. If the TCR recognises the class II MHC-antigen complex, signaling through the TCR causes a conformational change in the LFA-1 so that it binds with a higher affinity to the ICAM-1 and therefore stabilizes the association between the cells.
There also needs to be a secondary stimulus from the APC, called a co-stimulus, in order to activate the resting CD4 T Cell. The most important co-stimulus on a T cell is called CD28, which binds to either CD80 or CD86 molecules on APC cells (Figure 3). On recognition of both these stimuli, the T cell is now activated and can proliferate and differentiate. They are signaled to proliferate extensively by the cytokine interleukin 2 (IL-2). CD4 T cells both release IL-2 and express the IL-2 receptor so can work in an autocrine or paracrine (on antigen stimulated neighboring cells) manner. There is also a third signal in the form of different cytokines being released from APCs which can stimulate T helper cells to differentiate into different subtypes.
The medulla is less densely packed with cells. It mainly consists of lymph draining sinuses, which are separated by medullary cords. These medullary cords contain many antibody-producing plasma cells (mature B cells) as well as some memory T cells and macrophages. The plasma cells that remain in the medulla tend to be short lived and only secrete antibody, such as IgA or IgG, for a few weeks. This is in comparison to those plasma cells that are long lived as they migrate to the bone marrow and receive survival signals.
Lymphocytes are able to enter lymph nodes directly from the blood via specialised blood vessels called high endothelial venules (HEVs). These venules are called ‘high’ as they are made of cuboidal endothelial cells. HEVs are located in the paracortex of the lymph node, enabling the lymphocytes to enter the node in the T-cell area. B-cells then migrate to the cortex. This migration of B cells to the B cell area is due to the chemokine, B-Lymphocyte chemo-attractant (BLC). Follicular dendritic cells and stromal cells found in the B-cell area produce BLC. It binds to the CXCR5 chemokine receptor on B cells.
In order for lymphocytes to migrate out of the HEVs, the blood flow must be slow enough for the cells to be able to attach to the walls of the vessels. The slowing of the blood is due to vasodilation of vessels. Vasodilation occurs because the high endothelial venules that are the site of migration from blood to lymph nodes are located where small diameter capillaries become larger diameter venules. This diameter change slows the blood in the same way that vasodilation as a result of inflammation occurs. The slowing of the blood allows the lymphocytes to roll along the vessel wall. Cell adhesion molecules then govern the extravasation of lymphocytes into the lymph node via the same mechanism as the inflammatory response.
The whole lymph node is filled with a reticular meshwork. It is composed of fibroblastic reticular cells (FRCs), reticular fibres and fibrous extracellular matrix. This meshwork formed by the FRCs provides the structural framework for the node whilst also making spaces for motile immune cells to move through. It also may function as a physical barrier, keeping immune cells in their designated compartments, as well as preventing excessive growth or disordered interactions of cells.
Afferent lymphatic vessels bring the lymph fluid (fluid drained from tissues) into the lymph nodes at the side opposite the hilum. They empty their contents into the subcapsular space. The subcapsular space contains a mesh of reticular fibres, macrophages and dendritic cells. The lymph then flows through the cortex of the node. From the cortex, the lymph travels through sinuses to the paracortex and then to the medulla. It then travels through medullary cords to the efferent lymphatic vessel.
Lymph fluid leaves the lymph node via the efferent lymphatic vessel at the hilum of the node. The hilum is also the point for the entry and exit point of blood vessels and nerves. The lymph vessels from different nodes then join to eventually become 2 major lymphatic ducts, the thoracic duct and the right lymphatic duct. These then drain into venous circulation. In this manner, lymphocytes are continually being recirculated around the body. Lymphatic vessels contain valves to prevent the back flow of lymph, in the same fashion as venous circulation.
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