Lymph nodes and immune system

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

Each lymph node (nodus lymphaticus) is covered on the outside with a connective tissue capsule from which thin branches - crossbeams, capsular trabeculae - extend into the body. In the place where outgoing lymph vessels exit from the lymph node, the node has a slight depression - the hilum. In the region of the gates, the capsule thickens quite strongly, forming a collateral (hilar) thickening, more or less deeply penetrating into the node. From the portal thickening, the portal trabeculae leave the portal parenchyma. The longest of them connect with capsular trabeculae.

At the lymph nodes, to which lymph flows from the extremities (inguinal, axillary) and which are also called somatic, more often one gates, in visceral (mesenteric, tracheobronchial) - up to 3-4 gates. Through the gate into the lymph node enter the arteries, nerves, the veins come out and carry out lymphatic vessels.

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

Cells of the lymphoid tissue are located in the loops of the reticular stroma.

The parenchyma of the lymph node is divided into a cortical and brain substance. The cortex is darker on stained histological sections due to densely lying cellular elements, is closer to the capsule, occupying the peripheral parts of the node. The lighter medulla lies closer to the gates of the node and occupies the central part of it. In the cortex there are rounded formations 0.5-1.0 mm in diameter - the lymph nodes (noduli lymphoidei). Distinguish lymph node nodules without a breeding center and a breeding center (germinative center, centrum germinale).

A diffuse lymphoid tissue is located around the lymphoid nodules. In it, a cortical plateau is distinguished, which includes sections of the lymphoid tissue between the nodules - the interzell zone. The composition of the cortex plateau also includes tissue that is located outside of the lymphoid nodules, between them and the capsule. Inside the nodules, directly on the border with the brain substance, a strip of lymphoid tissue - a near-cortical substance, or thymus-dependent (paracortical) zone (paracortex, s.zona thymodependens), containing mainly T-lymphocytes, is allocated. In this zone there are pancapillary venules lined with cubic endothelium, through which the lymphocytes migrate to the bloodstream.

The parenchyma of the medulla is represented by the lymphoid tissue cords - the chordae medullares. They extend from the inner parts of the cortex to the lymph node and, together with the lymphoid nodules form a B-dependent zone. The flesh strands are connected to each other, resulting in complex interlacing.

The parenchyma of the lymph node is permeated with a dense network of narrow channels - lymphatic sinuses (sinus lymphatici), by which the lymph flowing into the node flows from the subcapsular sinus subcapsularis to the portal sinus. Along the capsular trabeculae lie the sinuses of the cortical (sinus corticales) and the medullary substance (sinus medullares). The latter reach the gates of the lymph node (collar thickening) and flow into the portal sinus located here. In the lumen of the sinuses, there is a shallow 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 specific features of lymph nodes

The lymph nodes, their stroma, develop from the mesenchyme near the forming plexus of the blood and lymph vessels, starting from the 5th-6th week of the embryo's life. Bookmarks of lymph nodes in different areas of the human body are formed at different periods up to and even after birth.

During the development of the node, the lumen of the lymphatic vessel becomes a subcapsular (marginal) sine. Intermediate sinuses develop on the basis of a branched lymphatic plexus, between the vessels of which the strands of embryonic connective tissue grow. Here cells of the lymphoid series are introduced. Starting from the 19th week, it is possible to see the border between cortical and cerebral substances in separate lymph nodes. Lymphoid nodules in the lymph nodes begin to form in the intrauterine period. Reproduction centers in lymphoid nodules appear shortly before birth and soon after. The basic age-forming processes in the lymph nodes end at 10-12 years.

Age changes in the lymph nodes (decrease in the number of lymphoid and growth of adipose tissue) are observed already in adolescence. The connective tissue grows in the stroma and parenchyma of the nodes, groups of fat cells appear. Simultaneously, the number of lymph nodes in the regional groups decreases. Many small-sized lymph nodes are completely replaced by a connective tissue and cease to exist as organs of the immune system. Nearby lying lymph nodes, often of medium size, fuse with each other and form larger nodes segmental or ribbon-shaped.

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

Vessels and nerves of lymph nodes

Each lymph node includes 1-2 to 10 small arterial branches from the nearest arteries. The venules that form from the capillaries merge into the veins, go to the organ gates and leave the node along with the outgoing lymph vessels.

Vegetative nerve fibers lymph nodes are obtained from plexuses located near the arteries, as well as from nerve trunks passing near the nodes.