Lymph nodes and the immune system

Lymph nodes (hodi lymphatici) are the most numerous organs of the immune system, serve as biological filters for the lymph (tissue fluid) flowing through them. They lie on the paths of lymphatic vessels from organs and tissues to lymphatic ducts and lymphatic trunks. Lymph nodes are usually located in groups. A group can have two or more, and sometimes several dozen nodes. For example, the group of superior mesenteric nodes has 66-404, axillary - 12-45, superficial inguinal - 4-20 nodes.

Each lymph node (nodus lymphaticus) is covered externally by a connective tissue capsule, from which thin branches - crossbars, capsular trabeculae (trabeculae) - extend into the organ. In the place where the efferent lymphatic vessels exit the lymph node, the node has a small depression - a gate (hilum). In the area of the gate, the capsule thickens quite strongly, forming a portal (hilar) thickening, more or less deeply protruding into the node. Portal trabeculae extend from the portal thickening into the parenchyma of the node. The longest of them connect with the capsular trabeculae.

Lymph nodes to which lymph flows from the extremities (inguinal, axillary) and which are also called somatic, usually have one gate, while visceral (mesenteric, tracheobronchial) nodes have up to 3-4 gates. Arteries and nerves enter the lymph node through the gates, and veins and efferent lymphatic vessels exit.

Inside the lymph node, between the trabeculae, is the reticular stroma. It is represented by reticular fibers and reticular cells, forming a three-dimensional network with loops of varying size and shape.

The cellular elements of lymphoid tissue are located in the loops of the reticular stroma.

The parenchyma of the lymph node is divided into the cortex and medulla. The cortex is darker on stained histological sections due to the densely packed cellular elements, is located closer to the capsule, and occupies the peripheral parts of the node. The lighter medulla is located closer to the gate of the node and occupies its central part. In the cortex there are rounded formations with a diameter of 0.5-1.0 mm - lymph nodes (noduli lymphoidei). A distinction is made between lymph nodes without a center of reproduction and with a center of reproduction (germintative center, centrum germinale).

Diffuse lymphoid tissue is located around the lymphoid nodules. It includes the cortical plateau, which includes areas of lymphoid tissue between the nodules - the internodal zone. The cortical plateau also includes tissue located outside the lymphoid nodules, between them and the capsule. Inside the nodules, directly on the border with the medulla, there is a strip of lymphoid tissue - the pericortical substance, or thymus-dependent (paracortical) zone (paracortex, s.zona thymodependens), containing mainly T-lymphocytes. In this zone are located cubical postcapillary venules lined with endothelium, through the walls of which lymphocytes migrate into the bloodstream.

The parenchyma of the medulla is represented by strands of lymphoid tissue - chordae medullares. They extend from the internal sections of the cortex to the gates of the lymph node and, together with the lymphoid nodules, form the B-dependent zone. The chordae medullares connect with each other, resulting in complex interweaving.

The parenchyma of the lymph node is penetrated by a dense network of narrow channels - lymphatic sinuses (sinus lymphatici), through which the lymph entering the node flows from the subcapsular (marginal) sinus (sinus subcapsularis) to the portal sinus. Along the capsular trabeculae lie the sinuses of the cortex (sinus corticales) and medulla (sinus medullares). The latter reach the gates of the lymph node (portal thickening) and flow into the portal sinus located here. In the lumen of the sinuses there is a finely looped network formed by reticular fibers and cells, in the loops of which foreign particles, dead and tumor cells can get stuck.

[ 1 ], [ 2 ], [ 3 ], [ 4 ]

Development and age-related features of lymph nodes

Lymph nodes and their stroma develop from the mesenchyme near the forming plexuses of blood and lymphatic vessels, starting from the 5th-6th week of embryonic life. Lymph node rudiments in different areas of the human body are formed at different periods up to birth and even after it.

During the development of the node, the lumen of the lymphatic vessel turns into a subcapsular (marginal) sinus. Intermediate sinuses develop on the basis of a branched lymphatic plexus, between the vessels of which strands of embryonic connective tissue grow. Cells of the lymphoid series settle here. Starting from the 19th week, in individual lymph nodes, one can see the emerging border between the cortex and medulla. Lymphoid nodules in the lymph nodes begin to form already in the intrauterine period. Reproduction centers in the lymphoid nodules appear shortly before birth and soon after it. The main age-related morphogenetic processes in the lymph nodes end by 10-12 years.

Age-related changes in the lymph nodes (reduction in the amount of lymphoid tissue and proliferation of adipose tissue) are observed already in adolescence. Connective tissue in the stroma and parenchyma of the nodes proliferates, and groups of fat cells appear. At the same time, the number of lymph nodes in regional groups decreases. Many small lymph nodes are completely replaced by connective tissue and cease to exist as organs of the immune system. Nearby lymph nodes, often medium-sized, grow together and form larger nodes of a segmental or ribbon-like shape.

[ 5 ], [ 6 ], [ 7 ], [ 8 ], [ 9 ], [ 10 ], [ 11 ]

Vessels and nerves of the lymph nodes

Each lymph node receives from 1-2 to 10 small arterial branches from the nearest arteries. The venules formed from the capillaries merge into veins, go to the organ gates and leave the node together with the efferent lymphatic vessels.

The lymph nodes receive vegetative nerve fibers from plexuses located near the arteries, as well as from nerve trunks passing near the nodes.