Lymph node

A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as filters or traps for foreign particles. They are important in the proper functioning of the immune system. They are packed tightly with the white blood cells called lymphocytes and macrophages.

Lymph nodes also have clinical significance. They become inflamed or enlarged in various conditions, which may range from trivial, such as a throat infection, to life-threatening such as cancers. In the latter, the condition of lymph nodes is so significant that it is used for cancer staging, which decides the treatment to be employed, and for determining the prognosis.

Lymph nodes can also be diagnosed by biopsy whenever they are inflamed. Certain diseases affect lymph nodes with characteristic consistency and location.

Function
The lymph nodes filter and monitor the lymph, an interstitial fluid, for foreign particles. These foreign particles commonly known as pathogens, or germs, can set up infections anywhere in the body. The lymph fluid inside of the lymph nodes contains lymphocytes, a type of white blood cell, which are continuously recirculated through the lymph nodes and the bloodstream. A germ, such as bacteria, will produce a chemical substance called an antigen which circulates through the bloodstream, the lymph system, and into lymph nodes. In response to the antigens, the lymphocytes in the lymph node make an antibody which will go out of the lymph node into circulation, seek, and destroy the infection producing the antigen. Other immune system cells will be made to fight the infection and "sent" to the lymph nodes. The increased numbers of immune system cells fighting the infection will make the node expand and become "swollen."

Structure
The lymph node is surrounded by a fibrous capsule, and inside the lymph node the fibrous capsule extends to form trabeculae. The substance of the lymph node is divided into the outer cortex and the inner medulla surrounded by the former all around except for at the hilum, where the medulla comes in direct contact with the surface.

Thin reticular fibers, elastin and reticular fibers form a supporting meshwork called reticular network (RN) inside the node, within which the white blood cells (WBCs), the most prominent ones being lymphocytes, are tightly packed as follicles in the cortex. Elsewhere, there are only occasional WBCs. The RN provides not just the structural support, but also will provide surface for adhesion of the dendritic cells, macrophages and lymphocytes. It allows for 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.

The number and composition of follicles can change especially when challenged by an antigen, when they develop a germinal center.

A lymph sinus is a channel within the lymph node lined by the endothelial cells along with fibroblastic reticular cells and allows for smooth flow of lymph through them. Thus, subcapsular sinus is a sinus immediately deep to the capsule, and its endothelium is continuous with that of the afferent lymph vessel. It is also continuous with similar sinuses flanking the trabeculae and within the cortex (cortical sinuses). The cortical sinuses and that flanking the trabeculae drain into the medullary sinuses, from where the lymph flows into the efferent lymph vessel.

Multiple afferent lymph vessels that branch and network extensively within the capsule bring lymph into the lymph node. This lymph enters the subcapsular sinus. The innermost lining of the afferent lymph vessels is continuous with the cells lining the lymph sinuses. The lymph gets slowly filtered through the substance of the lymph node and ultimately reaches the medulla. In its course it encounters the lymphocytes and may lead to their activation as a part of adaptive immune response.

The concave side of the lymph node is called the hilum. The efferent attaches to the hilum by a relatively dense reticulum present there, and carries the lymph out of the lymph node.

Cortex
In the cortex, the subcapsular sinus drains to trabecular sinuses, and then the lymph flows into the medullary sinuses.

The outer cortex consists mainly of the B cells arranged as follicles, which may develop a germinal center when challenged with an antigen, and the deeper cortex mainly consisting of the T cells. There is a zone known as the subcortical zone where T-cells (or cells that are mainly red) mainly interact with dendritic cells, and where the reticular network is dense.

Medulla
There are two named structures in the medulla:
 * The medullary cords are cords of lymphatic tissue, and include plasma cells, macrophages, and B cells
 * The medullary sinuses (or sinusoids) are vessel-like spaces separating the medullary cords. The Lymph flows into the medullary sinuses from cortical sinuses, and into efferent lymphatic vessels. Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells.

Shape and size
Human lymph nodes are bean-shaped and range in size from a few millimeters to about 1–2 cm in their normal state. They may become enlarged due to a tumor or infection, or inflamed due to leukemia. Lymphocytes, also known as white blood cells, are located within honeycomb structures of the lymph nodes. Lymph nodes are enlarged when the body is infected, primarily because there is an elevated rate of trafficking of lymphocytes into the node from the blood, exceeding the rate of outflow from the node, and secondarily as a result of the activation and proliferation of antigen-specific T and B cells (clonal expansion). In some cases, they may feel enlarged because of a previous infection; although one may be healthy, one may still feel them residually enlarged.

Lymphatic circulation
Lymph circulates to the lymph node via afferent lymphatic vessels and drains into the node just beneath the capsule in a space called the subcapsular sinus. The subcapsular sinus drains into trabecular sinuses and finally into medullary sinuses. The sinus space is criss-crossed by the pseudopods of macrophages, which act to trap foreign particles and filter the lymph. The medullary sinuses converge at the hilum and lymph then leaves the lymph node via the efferent lymphatic vessel towards either a more central lymph node or ultimately for drainage into a central venous subclavian blood vessel, most via the postcapillary venules, and cross its wall by the process of diapedesis.
 * The B cells migrate to the nodular cortex and medulla.
 * The T cells migrate to the deep cortex ("paracortex").

When a lymphocyte recognizes an antigen, B cells become activated and migrate to germinal centers (by definition, a "secondary nodule" has a germinal center, while a "primary nodule" does not). When antibody-producing plasma cells are formed, they migrate to the medullary cords. Stimulation of the lymphocytes by antigens can accelerate the migration process to about 10 times normal, resulting in characteristic swelling of the lymph nodes.

The spleen and tonsils are large lymphoid organs that serve similar functions to lymph nodes, though the spleen filters blood cells rather than lymph.

Distribution
Humans have approximately 500-600 lymph nodes distributed throughout the body, with clusters found in the underarms, groin, neck, chest, and abdomen.

Lymph nodes of the head and neck
Retropharyngeal: Drains lymph from the soft palate and the 3rd molars.
 * Cervical lymph nodes
 * Anterior cervical: These nodes, both superficial and deep, lie above and beneath the sternocleidomastoid muscles. They drain the internal structures of the throat as well as part of the posterior pharynx, tonsils, and thyroid gland.
 * Posterior cervical: These nodes extend in a line posterior to the sternocleidomastoids but in front of the trapezius, from the level of the Mastoid portion of the temporal bone to the clavicle. They are frequently enlarged during upper respiratory tract infections.
 * Tonsillar OR Sub-mandibular: These nodes are located just below the angle of the mandible, along the underside of the jaw on either side. They drain the tonsillar and posterior pharyngeal region, including the structures in the floor of the mouth and the maxillary anterior, bicuspid and 1st and 2nd molars. They also drain all of the mandibular teeth except the central incisors.
 * Sub-mental: These nodes are just below the chin. They drain the central incisors and midline of lower lip and tip of the tongue.
 * Supraclavicular lymph nodes: These nodes are in the hollow above the clavicle, just lateral to where it joins the sternum. They drain a part of the thoracic cavity and abdomen. Virchow's node is a left supraclavicular lymph node that receives the lymph drainage from most of the body (especially the abdomen) via the thoracic duct and is thus an early site of metastasis for various malignancies.

Lymph nodes of the thorax
The mediastinal lymph nodes along the esophagus are in tight connection with the abdominal lymph nodes along the esophagus and the stomach. That fact facilitates spreading of tumors cells through these lymphatics in cases of cancers of the stomach and particularly of the esophagus. Through the mediastinum, the main lymphatic drainage from the abdominal organs goes via the thoracic duct (ductus thoracicus), which drains majority of the lymph from the abdomen to the above mentioned left venous angle.
 * Lymph nodes of the lungs: The lymph is drained from the lung tissue through subsegmental, segmental, lobar and interlobar lymph nodes to the hilar lymph nodes, which are located around the hilum (the pedicle, which attaches the lung to the mediastinal structures, containing the pulmonary artery, the pulmonary veins, the main bronchus for each side, some vegetative nerves and the lymphatics) of each lung. The lymph flows subsequently to the mediastinal lymph nodes.
 * Mediastinal lymph nodes: They consist of several lymph node groups, especially along the trachea (5 groups), along the esophagus and between the lung and the diaphragm. In the mediastinal lymph nodes arises lymphatic ducts, which draines the lymph to the left subclavian vein (to the venous angle in the confluence of the subclavian and deep jugular veins).

Lymph nodes of the arm
These drain the whole of the arm, and are divided into two groups, superficial and deep. The superficial nodes are supplied by lymphatics that are present throughout the arm, but are particularly rich on the palm and flexor aspects of the digits.


 * Superficial lymph glands of the arm:
 * Supratrochlear glands: Situated above the medial epicondyle of the humerus, medial to the basilic vein, they drain the C7 and C8 dermatomes.
 * Deltoideopectoral glands: Situated between the pectoralis major and deltoid muscles inferior to the clavicle.


 * Deep lymph glands of the arm: These comprise the axillary glands, which are 20-30 individual glands and can be subdivided into:
 * Lateral glands
 * Anterior or pectoral glands
 * Posterior or subscapular glands
 * Central or intermediate glands
 * Medial or subclavicular glands

Lower limbs

 * Superficial inguinal lymph nodes
 * Deep inguinal lymph nodes
 * Popliteal lymph nodes

Pathology
Lymphadenopathy is a term meaning "disease of the lymph nodes." It is, however, almost synonymously used with "swollen/enlarged lymph nodes." In this case, the lymph nodes are palpable, and is a sign of various infections and diseases.