Last Updated on June 21, 2019
The lymph node is small, oval bean-shaped structure of lymphatic system formed of lymphoid tissue enclosed in a connective tissue capsule.
These occur in association with the lymphatic vessels and serve as filters for the blood. They are typically found concentrated near junctions of the major lymphatic vessels, most prominently in the neck, groin, and armpits.
The lymph nodes can get inflamed or enlarged in various conditions like infections and malignancies.
Lymph nodes are important in malignancy as their involvement is important for the staging of cancer.
Structure of Lymph Node
Lymph nodes are kidney-shaped structures ranging from the size of a few millimeters to about 1–2 cm.
Fibrous capsule is the outermost structure and surround the rest of the node substance and send trabeculations to the substance of the node.
The substance surrounded by the capsule is divided into the outer cortex and the inner medulla.
The cortex surrounds the medulla in most part except when the medulla is in direct contact with the hilus.
Thin reticular fibers and elastin form a supporting meshwork called reticulin inside the node. Lymph nodules are considered as working unit of the lymph node.
The nerve supply of the lymph node is through autonomic fibers. These supply the muscles in the blood vessels, the capsule and the trabeculae.
A detailed discussion follows.
Capsule
The capsule is the outermost part of the lymph node and is formed of dense irregular connective tissue and some collagenous fibers.
Trabeculae are an extension of the connective tissue inward throughout the lymph node.
These divide the space into compartments though incompletely.
Deep within the node, in the medullary portion, the trabeculae divide further and blend into the connective tissue of the hilum of the node.
Thus the capsule, the trabeculae, and the hilum make up the framework of the node within which the lymph sinuses are formed by connective tissue for circulation of lymphoid elements.
The trabecular spaces formed by the interlacing trabeculae contain the proper lymph node substance or lymphoid tissue but it does not completely fill the space.
There is a space between the outer margin of the lymphoid tissue and the enclosing trabeculae – a channel or space of uniform width throughout. It is called the subcapsular sinus and is defined as a space between the capsule and the cortex of the lymph node. It is also called the lymph path or lymph sinus.
Fine trabeculae of reticular connective tissue traverse the sinus.
Lymph goes from the subcapsular sinus into the cortical sinus toward the medulla of the lymph node.
Medullary sinuses are a broad network of lymph channels that drain toward the hilum of the node which then is passed to efferent vessels.
Cortex
The cortex is the part of the node below the capsule and the subcapsular sinus.
It has two parts, outer superficial part, and deeper part. The deeper part is also called paracortex and is more commonly known by that name only.
The lymph moves from subcapsular sinuses to trabecular sinus to the medulla.
Lymphoid nodules or follicles are islands of lymphoid tissues in the cortex and are arranged parallel to surface, often 2-3 layers deep. Thus the arrangement of lymphoid tissues is different from other parts of the node.
These follicles mainly contain B cells. These may develop a germinal center when they are presented with an antigen.
This germinal center is identified by the lighter staining it takes in the center. The germinal center is surrounded by a wider area packed with lymphocytes. This peripheral area is called cortical or primary nodule.
The germinal center is responsible for the formation of new lymphocytes by active cell division.
It is also called the reaction center because of the presence of macrophages in certain conditions.
Each of the follicles is pierced by a small blood vessel.
The lymph nodules or follicles are separated from the trabeculae and capsule by blood sinuses.
The paracortex mainly consists of the T cells which interact with dendritic cells.
The reticular network is quite dense in the paracortex.
Medulla
The medulla is not as dense as the cortex and also it does not contain lymphatic nodules.
The medulla contains large blood vessels, sinuses, and medullary cords.
Medullary cords are cords of lymphatic tissue which includes antibody-secreting plasma cells, B cells, and macrophages.
The medullary sinuses or sinusoids are vessel-like spaces separating the medullary cords. Lymph flows into the medullary sinuses from cortical sinuses, and into the efferent lymphatic vessel (which could be one or two in number).
Medullary sinuses contain histiocytes (stationary macrophages) and reticular cells.
Lymph sinuses of the medulla separate the trabeculae and lymph cords.
Hilus
It is a name for the depression on one side of the node which has a thick capsule and very thin medulla.
Hilus or hilum receives an artery, a vein, and an efferent lymph channel.
The blood vessels of the lymph node enter and leave the node at the hilus.
The artery after entering divides into arterioles which travel along the trabeculae and then further divides into capillaries which connect to venules.
Venules join repeatedly to form the main vein which leaves the node through the hilus.
The afferent lymphatic vessels which bring lymph to the node are numerous and enter the node through the whole of the outer surface after piercing the fibrous capsule.
Presence of valves in these vessels ensures lymph movement towards the node only.
After entering the node, the lymph vessels divide and move along the trabecular framework.
They unite to form the efferent vessel after traversing the whole node and draining it.
Flow of Lymph in the Node
Multiple afferent lymphatic vessels bring the lymph and enter the node on the convex side. This flows further through spaces called sinuses.
The sinuses are lined by endothelial cells along with fibroblastic reticular cells and allow for the smooth flow of lymph.
The endothelium of the subcapsular sinus is continuous with that of the afferent lymph vessel.
On the other side, it is continuous with the similar sinuses which flank the trabeculae and within the cortex.
All these sinuses drain the filtered lymph to the medullary sinuses from where it goes out of the node via efferent vessels at the hilum on the concave side.
Functions of Lymph Nodes
- Filtration
- Filter the lymph to identify the antigens and fight them.
- Temporarily arrests spread of cancer cells
- Trap and phagocytose foreign particles and poisonous substances
- Immune response
- Lymphocytes are white blood cells [B & T cells]. Some of them reside in the lymph node after they enter it.
- B cells are responsible for producing antibodies. After the enter the node, they go to lymphoid nodule or follicle and multiply. They are present in the nodular cortex or medulla. When presented with antigens from the afferent lymph, they produce antibodies.
- T cells are present in the deep cortex or paracortex. They carry out immunological responses by helping antibodies formation and in the development of immunity.
Clinical Significance
Lymph Node Swelling
LymhLymph nodes become enlarged in
- Infection
- Malignancy
- Autoimmune disease
- Foreign body reaction
Lymphadenopathy is the term used for swollen lymph nodes. A local cause would give rise to swelling of local lymph nodes whereas a more widespread condition would give rise to widespread lymphadenopathy.
Depending on the cause, a swollen lymph node could be painful or painless.
Sometimes, lymph nodes swelling could be the only presenting feature of cancer.
Lymphedema
This is swelling of the part due to fluid retention and tissue swelling. The cause is the absence of lymphatic drainage.
It can be congenital or secondary which occurs after removal of lymph nodes in surgery, often for malignancies.
Parasitic infections are also known to cause this.
Malignancy
The nodes could be primarily affected by cancer or may enlarge due to the spread of cancers from other parts.
Primary cancers of the lymph node are called lymphomas.
Lymph nodes involvement is often gauged to know the extent of tumor spread and also for the staging of cancer [TNM staging]
