Structure of the lymph node

A lymph node consists of capsule, reticulum, lymphoid tissue with lymphocytes, blood and nerve supplies (Figure 2).

The capsule is composed of dense collagenous connective tissue. It has a broad convex surface, and at one point, a deep indentation, the hilus. Afferent lymphatic vessels pass through the convex surface of the capsule and empty into the subcapsular (or marginal) sinus. One efferent lymphatic vessel leaves at the hilus where blood vessels and nerves enter and exit. Connective tissue projections of the capsule into the node are seen as the trabeculae.

The reticulum is a delicate meshwork enclosed by the capsule and trabeculae and consists of reticular cells and fibers. The reticular cells are large, branched fibroblasts which produce the reticular fibers.

Lymphocytes and other free cells lie in this meshwork and form the lymphoid tissue or lymph node parenchyma. Three distinct regions of the lymph node parenchyma extend from the capsule in towards the hilus. These are termed 'cortex', 'paracortex' and 'medulla'.

The cortex is largely occupied by lymphocyte aggregates forming nodules or follicles which are usually spherical or ovoid. Follicles may contain

Figure 1 The lymph node network. (Reproduced with permission from Roitt IM, Brostoff J and Male D (eds) (1998) Immunology, 5th edn. London: Mosby.)

tightly packed small lymphocytes and are termed 'primary follicles'. Alternatively, they may contain a central area consisting of larger lymphoid cells, macrophages and follicular dendritic cells. This central area is termed a 'germinal center' and the cuff of small lymphocytes that surround it forms the mantle zone. Nodules that develop a germinal center are termed 'secondary follicles'. Fully developed secondary follicles are composed of germinal center lymphocytes, mantle zone lymphocytes and mac-rophages/antigen-presenting (dendritic) cells. The germinal center is a site of active cell proliferation and cell death. A schematic model of the germinal center is shown in Figure 3. There are three major zones: a dark zone, a basal light zone and an apical light zone, and these are predominantly occupied by centroblasts, centrocytes and secondary blasts respectively. Primary B cell blasts which carry surface immunoglobulin receptors (slg+) enter the follicle and leave as memory B cells or antibody-forming cells (AFCs). Antigen-presenting follicular dendritic cells are mainly found in the two deeper zones, and cell death by apoptosis occurs mainly in the basal light zone where tingible body macrophages are also located. The lymphoid area between adjacent cortical follicles is called the 'interfollicular cortical tissue'.

The paracortex (or deep cortex) is a lymphoid area which is located immediately underneath the cortex, between the follicles and the medulla. Lymphocytes and lymphoblasts are densely packed in this area which displays a uniform lymphoid structure. Macrophages and antigen-presenting cells are interdispersed amongst the lymphocytes. Antigen-presenting cells are predominantly interdigitating cells similar to those found in the medulla of the thymic lobules. These cells, which belong to the monocyte/macrophage lineage, express membrane class II major histocompatibility complex (MHC) molecules at high density, have poor phagocytic ability and sparse lysosomes, but are very effective in presenting antigens to T cells. Other antigen-presenting cells are found in lymph vessels draining skin areas undergoing inflammation or antigenic stimulation. These are the 'veiled cells'. It appears that these veiled cells are I.angerhans cells, which migrate from the epidermis where they arc identifiable through the presence of Birbeck granules in their cytoplasm. Veiled cells reach the lymph node and seed into the paracortex where they lose their Birbeck granules and become interdigitating cells.

Another structural feature of the paracortex is the presence of high endothelial venules. These postcapillary venules are the sites where lymphocytes leave the circulation and enter the lymphoid tissue. The wall of these vessels is often infiltrated by small lymphocytes on their way from the blood into the parenchyma of the node. The lymph node medulla consists of cords of cells separated by lymphatic sinuses. Medullary cords are rich in plasma cells, macrophages and small lymphocytes.

Afferent lymphatic vessels enter the lymph node and empty into the large subcapsular sinus which lies directly beneath the capsule. Cortical sinuses run radially as extensions of the subcapsular sinus along the trabeculae, and continue between the medullary cords to converge into the terminal sinus, in the hilus region. One efferent iymphatic channel leaves the node emerging from the terminal sinus. Lymphatic sinuses are lined by endothelial cells and by macrophages termed sinus macrophages which become activated and increase in number during an inflammatory response.

Blood vessels enter the lymph node at the hilus. Arterioles reach the cortex through the trabeculae and break up into a rich capillary plexus. These ves

Figure 2 The structure of a lymph node. (Reproduced with permission from Roitt IM, Brostoff J and Male D (eds) (1998) Immunology, 5th edn. London: Mosby.)

sels group into venules, tributaries of veins which run from the cortex into the medulla and then leave the node via the hilus.

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