In this article we will discuss about the structure of lymph node with the help of suitable diagrams. Also learn about its functions.
Lymph node is small, oval or bean-shaped body. It is a collection of lymphoid tissue enclosed in a connective tissue capsule and lying along the lymphatic stream. From the capsule, strands of tissue pass into the substance of the node and are known as the connective tissue trabeculae.
In the human lymphatic nodes the trabeculae are ill-defined. They undergo repeated branching and along with the reticular tissue of the node, compose of ill-defined sinuses, the meshes of which are filled up with lymphoid cells in various stages of development. Histologically, the node can be divided into an outer part—the cortex, and an inner part—the medulla (Fig. 5.5).
Cortex:
The lymphoid tissues remain scattered throughout the node but in the cortex they are found in especially collected islands—the lymphoid nodules or follicles which vary from 0.35 to 1.0 mm in diameter. These nodules remain arranged parallel to the surface, sometimes two or three layers deep.
Each lymph nodule is pierced by a small blood vessel and shows a less dense area at its centre, which takes a lighter stain. Surrounding this lighter zone there is a wider area packed with lymphocytes. The central zone (lighter) is called the germinal centre or secondary nodule, whereas the peripheral area is called the cortical nodule or primary nodule.
These lymph nodules are separated from the trabeculae and capsule by blood sinuses. The germinal centre is regarded as the area in which new lymphocytes are being formed by active cell division. Numerous macrophages are found in the germinal centre under certain pathological conditions and for this reason this central zone is called as reaction centre.
Medulla:
The medulla of the lymph node is much less dense than the cortex and is devoid of lymphatic nodules. It consists of scattered lymph cells, different varieties of reticuloendothelial cells and sometimes a few multi-nucleated giant cells (Fig. 5.6).
The cytological arrangement of the trabeculae in the medulla is irregular and frequently anastomosing lymph cords of it run through the communicating divisions surrounded by the meshes of the trabeculae. Lymph sinuses of the medulla separate the trabeculae and lymph cords. In some animals, such as ox, large pigmented branching cells are found in the medulla, which give it a darker colour.
Hilus (Hilum):
At one side of the node there is a depression, known as the hilus were the capsule is thickened. Here the cortical part becomes very much thin and the medulla comes to the surface. At the hilus the node is pierced by three vessels—an artery, a vein, and the efferent lymph channel.
The artery after entering the node breaks up into numerous arterioles which proceed at first along the trabeculae and then each arteriole breaks up into a bunch of capillaries which becomes surrounded by masses of lymphatic cells. The capillaries are then collected into venules, which join repeatedly to form the main vein that leaves the node through the hilus.
Thus the blood vessels of the lymph node enter and leave the node at the hilus. But this is not the case with the lymphatics. The afferent lymphatic vessels enter the node through the whole of the outer surface by piercing the fibrous capsule, and have valves opening toward the node. These afferent vessels are generally thinner and more numerous than the efferent vessels.
After entering the node the lymph vessels undergo repeated branching, the branches proceed along the trabecular framework and after traversing and draining the whole of the lymph node they converge to form the main efferent lymph channel, which passes out of the node at the hilus, and possess valves opening outward from the node. Thus the afferent lymph vessels enter the capsule in as scattered distribution and penetrate the subscapular sinuses. Efferent vessels leave the node through the hilus (Fig. 5.5).
The nerves supplying the lymph nodes are non-medullated autonomic fibres, which are distributed to the plain muscles in the blood vessels, the capsule and the trabeculae.
Haemal Lymph Node:
In many animals some lymph nodes are found to possess red colour, due to the presence of blood in them. In man they are found in the retroperitoneal tissues and also in the mediastinum. In these nodes some of the so-called lymphatic channels contain blood, while the rest of the nodes possess the same structure as the typical lymph node. Spleen may be regarded as the modified haemal lymph (haemolymph) node.
Function of Lymph Nodes:
i. They produce and supply lymphocytes to the blood and as a supportive function the trabeculae carry blood vessels which supply the node.
ii. They make screening of the lymph by means of phagocytic activity.
iii. They serve a great defensive role against bacterial infections (Fig. 5.5).
iv. They temporarily stop the spread of cancer cells as those cells have to penetrate through the lymph vessels to the lymph nodes from where they spread in the body.
v. They act as mechanical filters to resist the entrance of poisonous substances into circulation.
vi. They carry out immunological responses. They help in elaboration of antibodies and in the development of immunity.
vii. Lymph nodes produce ϒ-globulin.