Lymph refers to a transparent white fluid traveling via the vessels, flowing within tissues, and functioning well to clean all body parts. After moving through the lymphatic system, the fluid drains into the nodes. The lymphatic system is an intricate network of thin vessels, organs, ducts, valves, and nodes. All of which defend and maintain the fluid milieu of the body by generating, filtering, and supplying lymph as well as by forming diverse blood cells.
Introduction to Lymph Nodes
Lymph nodes refer to small glands of the immune system, which are spread across the body along lymphatic system and appear as small bulbs. Recognized as the battalion of B, T, and similar immune cells. Lymph nodes work like filters along the system to trap the foreign particles, toxins, cancer cells, as well as germs such as viruses and bacteria so that they can be removed from the body. The nodes are totally filled with the white blood cells called macrophages and lymphocytes and are essential for smooth functioning of the immune system. While the lymphatic system spreads the lymph fluid up to the cells and tissues, the lymph nodes themselves are the garrisons of various types of tissues and cells, of which lymphoid tissues and white blood cells are copious.
Lymph nodes contribute drastically to the body’s defense power against infection. Most of the people are aware of the fact that the swollen or tender lymph nodes is the most common symptom of infection via cold or sore throat. Because these nodes tend to get inflamed or enlarged in different medical conditions, they carry clinical significance. Moreover, the condition of lymph nodes is also used for cancer staging due to which the treatment can be decided. They are also diagnosed through biopsy in case they are inflamed. In short, whether trivial or major, a few chronic diseases have an impact on the lymph nodes.
Structure of the Lymph Nodes
Starting from the basic structural concepts of shape and size, lymph nodes in humans possess a bean shape and tend to range from some millimeters to one-two centimeters in size when they are normal. Lymphocytes, that are a kind of white blood cells, reside inside the honeycomb arrangement of the nodes. When these cells are trafficking from the bloodstream into the node at an abnormally high rate such that the outflow rate from the node is exceeded, the nodes tend to enlarge, showing a sign of infection. Enlargement can also occur due to the activation and explosion of antigen-specific B and T cells. In exceptional cases, the nodes may be enlarged due to a former infection even though you are healthy.
There are nearly 600-700 lymph nodes in the body, each of which is flanked by a fibrous capsule that penetrates inside the node for forming trabeculae – a mix of connective tissue and small amounts of plain muscle fibers. While outer structure of a node is made up of capsule, the inner structure of a node is split into several parts: Medulla, reticular network, and cortex. Medulla is flanked by the cortex all over, except at the hilum at which it directly meets the surface. Structurally, each lymph node is composed of the discussed parts below.
Located at the edge, the capsule is composed of collagen as well as a sub-capsular sinus. The fluid flows into this sinus there exists abundant lymphocytes, dendritic cells, and antigen processing macrophages.
A capsule covers each node. The profound surface of this capsule is the site from where several trabeculae lengthen radially towards the node’s interior, where they tend to be continuous with the minute reticulum. This is where a reticular network exists as a supporting meshwork of thin reticular fibers and elastin. It is in this network that the white blood cells, more prominently the lymphocytes, are compactly filled as follicles in the cortex. The composition as well as the quantity of follicles tends to change, particularly when confronted by an antigen. At other places, the white blood cells exist as occasional white blood cells only. The network facilitates the desired structural support, adhesion of the dendritic cells and lymphocytes to the surface, exchange of material with blood, and growth as well as regulatory factors to activate and develop immune cells.
Beneath the capsule there exists the sub-capsular sinus surrounding the node, with the site of hilum as the exception. Edged by the endothelial cells as well as fibroblastic reticular cells, the sinus channel facilitates smooth flow of the lymph fluid through the nodes. It is immediately deep to the capsule and its endothelium area is incessant with the outer afferent lymph vessels as well as with similar other sinuses flanking the trabeculae as well as the cortical sinuses inside the cortex. There are many afferent vessels, which tend to unlock themselves into the sub-capsular sinus. The cortical sinuses along with the ones surrounding the trabeculae empty themselves into the medullary sinuses, the site from where the fluid reaches the efferent lymph vessels. The fluid filters through the reticulin fibers and departs the node via an efferent vessel.
Several afferent lymph vessels forming an extensive network within the capsule fetch the lymph fluid into the node where it first enters the sub-capsular sinus. The afferent lymph vessels’ innermost lining is continuous with the cells edging the lymph sinuses. Gradually, the lymph fluid is filtered via the node and ultimately moves towards the medulla. On the way, the lymph meets lymphocytes and may activate them to trigger adaptive immune response.
This is the concave side of the node. It is called the hilum to which the efferent vessels attach via a relatively thick reticulum to carry the fluid out of the node.
The sub-capsular sinus pumps out to the trabecular sinuses from where the fluid flows into the medullary sinuses. The outer area of the cortex holds the B cells prearranged as follicles that may form a germinal center, while the inner cortex holds the T cells. In addition, there exists a sub-cortical zone where T-cells (red cells) intermingle with the dendritic cells in the area where the reticular network is thick. The area where T as well as accessory cells exists is known as paracortex (inner cortex). The cortex, in short, is split into three zones.
- First Zone: Has loosely filled small lymphocytes, rare plasma cells, and macrophages in the follicle’s periphery and lengthens into the cords of medulla.
- Second Zone: Has more tightly packed macrophages and small lymphocytes and restricted to cortical as well as paracortical (inner cortex) regions.
- Third Zone: Has the germinal centre that holds big macrophages and lymphocytes.
As per the allotment of T- and B-lymphocytes, the cortex has two parts:
- Outer: Holding undeveloped B-lymphocytes
- Inner: Holding T-lymphocytes and existing between the medulla and the germinal centre (also called thymus dependent zone or paracortex)
This component is made up of two sub-structures: Cords and sinuses. The medullary cords of the lymphatic tissue contain the B cells, macrophages, and plasma cells secreting antibodies. On the other hand, the medullary sinuses refer to vessel-like areas that separate the cords and contain reticular cells and immobile macrophages. The fluid flows into these sinuses from the cortical sinuses, and then reaches the efferent lymphatic vessels. In short, the medulla is the hub of sinuses, cords, and blood vessels.
At the hilum, the arteries enter and split themselves into branches that reach within the trabeculae. The arteries, within the cortex, further split to form cloisters of capillaries and arterioles with several loops. Capillaries lead to veins and venules, which go back to meet the hilum. They are more in number near the follicles, while the post capillary venules are copious in the paracortex.
Functions of the Lymph Nodes
In general, the lymph nodes operate as an organic filtering system as well as an antibody producer.
- Lymph Filtration: Lymph, the white fluid, is drained and gathered from the adjacent parts of the nodes. In specific terms, the afferent lymphatic vessels supply unfiltered body fluid to the node for filtration. Therefore, the lymph nodes behave as filters for disease-invoking culprits such as bacteria and viruses or foreign particles, which are collected in the nodes and destroyed by the white blood cells. The fluid is filtered and collected by the efferent vessels that deliver it to the cardiovascular system.
- Antibody Production: Lymph nodes are the major areas where autoimmune responses are triggered. They possess lymphocytes that tend to produce antibodies in response to the antigen, during the infection. These antibodies then target the pathogens and slay them. This leads to a rise in the production of beta cells that finally results in inflamed, sensitive, and painful nodes. When the body is invaded by foreign agents, the aching swelling being felt in the armpits, neck, tonsils, and groin is due to the microorganisms trapped within the lymph nodes. That is why one gets to observe a considerable change in the node size during mild infection such as sore throat or serious condition like cancer.
Circulation of Lymph Nodes
The lymph fluid circulates to the node through the afferent vessels and drains inside in the subcapsular sinus that, in turn, drains into the trabecular and medullary sinuses. The sinus is interlaced by the macrophages, which trap the pathogens and foreign particles for triggering the function of filtering. The medullary sinuses meet at the hilum and the fluid then departs from the node through the efferent vessel either towards a more central node or for reaching a central venous sub-clavian vessel through post capillary venules.
When an antigen is sensed by a lymphocyte, the B cells are activated for migrating to the germinal centers. On the other hand, when the plasma cells producing antibodies are created, they migrate to the cords in the medulla. Antigens when spur the lymphocytes tend to speed up the migration process up to 10 times due to which the nodes swell.
Location of the Lymph Nodes
- Cervical Nodes: Present mainly in the head and neck region. They are usually located deep in the neck, at the jaw’s bottom, and behind the ears. There are six nodes in the cervical area, all of which fetch the fluid from the face, scalp, pharynx, and nasal cavity.
- Axillary Nodes: Present in the armpit region and are of two types: Superficial and deep. They take lymph from walls of breast and thorax as well as from the arm.
- Supraclavicular Nodes: Present along the clavicle or collarbone and take lymph from the upper parts of the chest as well as of the collarbone.
- Mediastinal Nodes: Present in the chest’s central part, perhaps between the lungs. They fetch lymph from the chest cavity’s middle section, lungs, and the upper abdomen area.
- Supratrochlear Nodes: Present on the area just over the joint of elbow. They fetch lymph from fingers, shallow arm areas, and from the hand on the arm’s ulnar side.
- • Mesenteric Nodes: Spread across in the lower abdomen and are present near the small intestine. They fetch lymph from colon, jejunum, and rectum’s upper part.
- • Inguinal Nodes: Present in the groin area and are of two types that are same as for axillary lymph nodes. They obtain the lymph fluid from the buttock, genital areas, anus, legs, and abdominal wall.
- • Femoral Nodes: Present in the upper thigh area as well as along the femoral veins and are just below the inguinal nodes. They fetch lymph from a few genital parts, thighs, buttocks, and the leg’s medial side.
- • Popliteal Nodes: Present in the popliteal fossa region of the knee and are distributed in two sets. A few reside beneath the popliteal fascia area, while the rest are located between the posterior knee joint surface and popliteal artery. The lymph is fetched from the feet, calf, thighs, and knees.
Overall, lymph nodes play a very vital role in strengthening the immune system as well as in diagnosing some trivial to serious medical conditions.