Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Organs of Hematopoiesis and the Immune System. Lymphatic System
Peripheral Organs of the Immune System
Lymph Nodes

Lymph nodes act as biological filters along the pathways of lymph drainage from Organs and Tissues toward lymphatic ducts and trunks, which empty into the large Veins in the lower neck. Together with lymphatic capillaries, vessels, trunks, and ducts, lymph nodes are an integral part of The Immune System, providing immune defense for virtually all parts and organs of The Human Body where lymphatic capillaries ramify. Lymph is filtered through the lymph nodes; it is essentially tissue fluid that has been absorbed into the lymphatic capillaries and contains various dissolved and suspended substances, Metabolic waste products, and debris such as dead Cell fragments and dust particles. Within the lymph nodes, these particles—including microbial agents and even tumor Cells (in oncological diseases)—are trapped. Lymphocytes recognize the foreign nature of these particles and destroy them with the help of macrophages. Dust particles, including tobacco dust from the Lungs, settle within the lymphoid tissue of the lymph nodes, impairing their function and potentially rendering them dysfunctional.

Each lymph node is approached by 4–6 or more afferent Lymphatic vessels, whose walls fuse with the lymph node capsule, and their lumen continues into the subcapsular sinus. After passing through the lymph node, the now-purified lymph exits via 2–4 efferent lymphatic vessels, which proceed either to the next lymph node in the same or an adjacent group, or to a major collecting vessel—a lymphatic trunk or duct.

Lymph nodes are arranged in groups consisting of two or more nodes. Sometimes the number of nodes in a group reaches several dozen. The dimensions of lymph nodes are highly variable, ranging from 0.5–1 mm to 50–75 mm. The nodes are ovoid, spherical, or bean-shaped, though larger ribbon-shaped and segmental forms may also occur.

Every lymph node is enclosed in a Connective Tissue capsule from which trabeculae (partitions) of varying lengths extend into the interior of the node. At the site where efferent lymphatic vessels leave the node, There is a small depression known as the hilum. The artery and nerves enter the node through the hilum, while the vein and lymphatic vessels exit. In the region of the hilum, the capsule is thickened, forming the hilar thickening, which projects more or less deeply into the interior of the node. Trabeculae also extend from the hilar thickening into the parenchyma of the lymph node.

Inside the lymph node, between the trabeculae, lies a network of reticular fibers and cells. Cells of the lymphoid series (lymphocytes, plasma cells) and macrophages are located within the meshes of the reticular tissue. The parenchyma of lymph nodes is divided into the cortex and medulla (Fig. 75). The cortex, located closer to the capsule of the node, is darker due to densely packed cellular elements. The lighter medulla lies closer to the hilum and in the central part of the node. The cortex contains spherical lymphoid nodules, 0.5–1 mm in diameter, which represent accumulations of lymphoid cells, predominantly B lymphocytes. Situated between the lymphoid nodules is a diffuse lymphoid tissue—the so-called cortical plateau with a more or less uniform distribution of cells. Internal to the lymphoid nodules, directly at the border with the medulla, lies a band of lymphoid tissue known as the Thymus-dependent paracortical zone (paracortex), which contains predominantly T lymphocytes.

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Fig. 75. Structure of a lymph node:

1 — capsule; 2 — capsular trabecula; 3 — afferent lymphatic vessel; 4 — subcapsular (marginal) sinus; 5 — cortex; 6 — paracortical (thymus-dependent) zone (paracortex); 7 — lymphoid nodule; 8 — germinal center; 9 — perinodular cortical sinus; 10 — medullary cord; 11 — medullary sinus; 12 — hilar sinus; 13 — efferent lymphatic vessel; 14 — hilar thickening; 15 — Blood vessel

The parenchyma of the medulla is represented by cords of lymphoid tissue—medullary cords—which extend from the inner Regions of the cortex to the hilum of the lymph node. The medullary cords interconnect to form complex networks and serve as sites of B lymphocyte accumulation (similar to the lymphoid nodules). Plasma cells and macrophages are also found within the medullary cords.

The cortex and medulla of the lymph node are penetrated by a dense network of narrow channels—lymphatic sinuses—whose walls are formed by flattened endothelial cells. Through these sinuses, the lymph entering the node flows from the subcapsular (marginal) sinus toward the hilar sinus. The subcapsular sinus is located directly beneath the capsule of the node, between the capsule and the parenchyma. It receives afferent lymphatic vessels carrying lymph either from an organ for which this node is regional, or from a preceding lymph node. From the subcapsular sinus, intermediate sinuses of the cortex and medulla extend along the trabeculae. The latter reach the hilum of the lymph node, where they drain into the hilar sinus. The hilar sinus gives rise to efferent lymphatic vessels leading either to the next "filter"—a lymph node—or to ducts and trunks.

Through the thin walls of the sinuses, lymphocytes, macrophages, and other actively migrating cells can easily pass back and forth between the lymphoid tissue of the cortex and medulla and the lymph.

The lumen of the sinuses contains a fine-meshed network formed by reticular fibers and cells. The meshes of this network trap cell debris brought into the lymph node with the lymph, as well as foreign particles (such as coal and tobacco dust in nodes regional to the respiratory organs), microbial bodies, and tumor cells. Dust particles are transported by macrophages into the parenchyma of the node, where they are deposited. Remnants of destroyed cells entering the lymph stream are destroyed, while tumor cells may initiate a secondary tumor (metastasis) within the lymph node.

In humans, starting from adolescence and maturity, especially in old age, as well as in smokers and individuals working without protection in dusty environments, the lymphoid parenchyma of lymph nodes is progressively replaced by adipose tissue. Consequently, the Protective Functions of lymph nodes, like those of other Organs of the immune system, decline.

Review and Self-Assessment Questions:

1. Describe the functions performed by the immune system in the human body.

2. Name the central and Peripheral Organs of the Immune System.

3. Explain which organs of the immune system contain lymphoid nodules and describe The structure of these nodules.

4. What do you know about the Tonsils of the lymphoid ring? Where is each tonsil located?

5. Describe the STRUCTURE AND FUNCTIONS of the lymphoid structures of the spleen.

6. Describe the structure and functions of a lymph node.