A lymph node, or lymph gland, is a kidney-shaped organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that include B and T cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles including cancer cells, but have no detoxification function.

In the lymphatic system, a lymph node is a secondary lymphoid organ. A lymph node is enclosed in a fibrous capsule and is made up of an outer cortex and an inner medulla.

Lymph nodes become inflamed or enlarged in various diseases, which may range from mild throat infections to life-threatening cancers. The condition of lymph nodes is very important in cancer staging, which decides the treatment to be used and determines the prognosis. Lymphadenopathy refers to glands that are enlarged or swollen. When inflamed or enlarged, lymph nodes can be firm or tender.

Structure

thumb|Cross-section of a lymph node with sections labelled.1) Capsule; 2) Subcapsular sinus; 3) [[Germinal center; 4) Lymphoid nodule; 5) Trabeculae|alt=]]

Lymph nodes are kidney or oval shaped and range in size from 2 mm to 25 mm on their long axis, with an average of 15 mm.

Each lymph node is surrounded by a fibrous capsule (made of collagenous connective tissue), which extends inside the lymph node to form trabeculae. The substance of a lymph node is divided into the outer cortex and the inner medulla. The hilum is an indent on the concave surface of the lymph node where lymphatic vessels leave and blood vessels enter and leave.

There are no lymph nodes in the central nervous system, which is separated from the body by the blood–brain barrier. Lymph from the meningeal lymphatic vessels in the CNS drains to the deep cervical lymph nodes. However, the CNS does innervate lymph node by sympathetic nerves. These regulate lymphocyte proliferation and migration, antibody secretion, blood perfusion, and inflammatory cytokine production.

Size

{|class="wikitable"

|+Upper limit of lymph node sizes in adults

|-

| Generally || 10 mm

|-

| Inguinal || 10 – 20 mm

|-

| Pelvis || 10 mm for ovoid lymph nodes, 8 mm for rounded

|-

| Jugulodigastric lymph nodes || 11mm

|-

!colspan=2| Upper abdominal

|-

| Retrocrural space || 6 mm

|-

| Paracardiac || 8 mm The outer cortex consists of groups of mainly inactivated B cells called follicles.

The medulla contains large blood vessels, sinuses and medullary cords that contain antibody-secreting plasma cells. There are fewer cells in the medulla.

As part of the reticular network, there are follicular dendritic cells in the B cell follicle and fibroblastic reticular cells in the T cell cortex. The reticular network provides structural support and a surface for adhesion of the dendritic cells, macrophages and lymphocytes. It also allows exchange of material with blood through the high endothelial venules and provides the growth and regulatory factors necessary for activation and maturation of immune cells.

Lymph flow

thumb|alt=Human lymph node|Labeled diagram of human lymph node showing the flow of [[lymph]]

thumb|Afferent and efferent vessels

Lymph enters the convex side of a lymph node through multiple afferent lymphatic vessels, which form a network of lymphatic vessels () and flows into a space () underneath the capsule called the subcapsular sinus. Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells, the former of which, along with T and B cells, become activated in the presence of antigens through lymphatic flow. The fewer efferent vessels allow this flow to be slowed, providing time to activate and distribute a larger number of immune cells in the event of an infection.

A lymph node contains lymphoid tissue, i.e., a meshwork or fibers called ' with white blood cells enmeshed in it. The regions where there are few cells within the meshwork are known as '. It is lined by reticular cells, fibroblasts and fixed macrophages.

Capsule

thumb|right|Lymph node tissue showing trabeculae

Thin reticular fibers (reticulin) of reticular connective tissue form a supporting meshwork inside the node. The lymph node capsule is composed of dense irregular connective tissue with some plain collagenous fibers, and a number of membranous processes or trabeculae extend from its internal surface. The trabeculae pass inward, radiating toward the center of the node, for about one-third or one-fourth of the space between the circumference and the center of the node. In some animals they are sufficiently well-marked to divide the peripheral or cortical portion of the node into a number of compartments (nodules), but in humans this arrangement is not obvious. The larger trabeculae springing from the capsule break up into finer bands, and these interlace to form a mesh-work in the central or medullary portion of the node. These trabecular spaces formed by the interlacing trabeculae contain the proper lymph node substance or lymphoid tissue. The node pulp does not, however, completely fill the spaces, but leaves between its outer margin and the enclosing trabeculae a channel or space of uniform width throughout. This is termed the subcapsular sinus (lymph path or lymph sinus). Running across it are a number of finer trabeculae of reticular fibers, mostly covered by ramifying cells.

Inverted lymph nodes

Some mammal species, such as pigs, rhinoceroses, hippopotamuses, and certain cetaceans, have so-called "inverted" lymph nodes. In these nodes, the afferent lymph carries antigens from the center (where the B-cell follicles are located) toward the periphery. Mature B and T lymphocytes exit the lymph node from its periphery directly into the general bloodstream via efferent venules. These structural differences do not appear to impact the functionality of the lymph nodes.

Function

In the lymphatic system, a lymph node is a secondary lymphoid organ. Features of the medical history may point to the cause, such as the speed of onset of swelling, pain, and other constitutional symptoms such as fevers or weight loss. For example, a tumour of the breast may result in swelling of the lymph nodes under the arms and weight loss and night sweats may suggest a malignancy such as lymphoma.

In addition to a medical exam by a medical practitioner, medical tests may include blood tests and scans may be needed to further examine the cause. A biopsy of a lymph node may also be needed.

Cancer

thumb|200px|right|[[Micrograph of a mesenteric lymph node with adenocarcinoma]]

Lymph nodes can be affected by both primary cancers of lymph tissue, and secondary cancers affecting other parts of the body. Primary cancers of lymph tissue are called lymphomas and include Hodgkin lymphoma and non-Hodgkin lymphoma. Cancer of lymph nodes can cause a wide range of symptoms from painless long-term slowly growing swelling to sudden, rapid enlargement over days or weeks, with symptoms depending on the grade of the tumour. Most lymphomas are tumours of B-cells. Lymphoma is managed by haematologists and oncologists.

Local cancer in many parts of the body can cause lymph nodes to enlarge because of tumorous cells that have metastasised into the node. Lymph node involvement is often a key part in the diagnosis and treatment of cancer, acting as "sentinels" of local disease, incorporated into TNM staging and other cancer staging systems. As part of the investigations or workup for cancer, lymph nodes may be imaged or even surgically removed. If removed, the lymph node will be stained and examined under a microscope by a pathologist to determine if there is evidence of cells that appear cancerous (i.e. have metastasized into the node). The staging of the cancer, and therefore the treatment approach and prognosis, is predicated on the presence of node metastases.

Lymphedema

Lymphedema is the condition of swelling (edema) of tissue relating to insufficient clearance by the lymphatic system. It can be congenital as a result usually of undeveloped or absent lymph nodes, and is known as primary lymphedema. Lymphedema most commonly arises in the arms or legs, but can also occur in the chest wall, genitals, neck, and abdomen. Secondary lymphedema usually results from the removal of lymph nodes during breast cancer surgery or from other damaging treatments such as radiation. It can also be caused by some parasitic infections. Affected tissues are at a great risk of infection. Management of lymphedema may include advice to lose weight, exercise, keep the affected limb moist, and compress the affected area. Furthermore, the tonsils filter tissue fluid whereas lymph nodes filter lymph.

See also

  • Peyer's patch
  • Lymph sacs

References

Bibliography

  • Lymph Nodes
  • Lymph Nodes Drainage
  • An overview of Normal Lymph Nodes and Swollen lymph nodes and their evaluation