LYMPH NODE

Lymph nodes (noduli lymphatici) are the smallest lymphatic organs associated with a network of lymphatic vessels where they act as a lymph filter. All lymph passes through at least one lymph node before returning to the venous system via the thoracic duct. The lymph from the adjacent draining area is delivered to one node through a number of afferent lymphatic vessels that perforate the nodal capsule (Fig. 1A, B) on the convex surface. Inside the node, the lymph enters the sinus system: The first sinus the lymph enters is the subcapsular sinus (between the capsule and the cortex); from there it reaches the paratrabecular sinus (along the trabecula; since this sinus passes through the cortex between the lymphatic follicles, it is also referred to as the perifollicular or intermediate sinus). From there, the lymph passes into the medullary sinus and is then discharged through the efferent lymphatic vessel out of the lymph node. The construction of the subcapsular sinus is shown in Fig. 1 B. Within the sinus, the reticular (connective tissue) cells are oriented perpendicularly to a surface of the sinus. Cells and other substances that retained during filtration are removed from the lymph by phagocytosis. Resident macrophages send their processes to the lymph sinus. Phagocytosis of material within the lymph node is remarkably effective: almost all phagocytic material is removed from the lymph after a single passage through the lymph node. Macrophages present the antigens to lymphocytes and initiate an immune response.

A surface of the lymph node is covered by a sheath of dense collagen connective tissue from which trabeculae arise. Due to a small size of the nodes (only a few mm), the trabeculae are also tiny. Reticular connective tissue, which form a supportive framework of the node (and other secondary lymphatic organs), are attached to these stromal components. The lymph node parenchyma can be characterized as a lymphoreticular organ. Parenchyma is divided into cortex and medulla. The cortex contains lymphatic nodules, which are most often arranged in a single layer. Nodules that arise as aggregations of B lymphocytes are referred to as the primary lymphatic nodules. In response to antigen stimulation, a bright germinal centre (formed by lymphoblasts) appears within the nodules. Such nodules are referred to as the secondary follicles. Evenly spread T lymphocytes between and below the nodules are described as the inner (non nodular) cortex or paracortex. The medulla contains medullary sinuses, among which are placed lymphatic (medullary) cords. Medullary sinuses are relatively wide and mostly bright (the colour depends on the number of cells inside the sinus). Medullary cords form irregularly branched strands of lymphatic tissue consisting of lymphocytes, plasma cells and resident macrophages.

Fig. 1A (left). Light microscopic structure of the lymph node. The capsule of the dense connective tissue (brown) is perforated by many afferent lymphatic capilaries (A). This way a lymph enters a system of sinuses (a flow is indicated by the arrow). A cleansed lymph is drained from the hilus by efferent lymphatic vessel (green); arteriole (red) enters and venule (blue) leaves the lymph node in the same site. Fig. 1B (right). Subcapsular sinus. Afferent lymphatic capilary lined by the endothelium (green) penetrates the capsule (brown). Processes of reticular connective tissue cells (orange) and reticular fibres span the sinus (*) and form a complex network across to which the lymph is filtered. Walls of the sinus are lined by litoral cells (yellow) that are derived from flattened reticular cells (this lining is discontinuous and lacks the basal lamina). Macrophages (Ma) present an antigen to lymphocytes (Ly). Author of the imageo R. Krstic; colorized by V. Vrbacká and J. Mokrý.