Structure of Lymph Nodes

Table of Contents

What are Lymph Nodes?

Structure

Capsule

Subcapsular Sinus

Cortex

Medulla

Cells of the Lymph Node

Flow of Lymph

Function of Lymph Nodes

Clinical Significance

Frequently Asked Questions

What are Lymph Nodes?

Lymph nodes are small, bean-shaped organs found in the lymphatic and adaptive immune system. They are clusters of lymphoid tissue encapsulated in connective tissue layers and associated with lymphatic vessels.

The lymphatic system that is composed of lymphatic vessels and lymphatic organs, functions to filter and drain the interstitial fluid arising from end-stage capillaries. The lymph nodes are found at the convergence of major blood vessels and any adult human has about 800 lymph nodes mostly concentrated in the groin, armpit, neck and abdominal regions.

The lymph nodes serve as the secondary lymphoid organ in the lymphatic system. They are the major sites for lymphocytes such as B-cells and T-cells. They are important for proper functioning of the immune system as they help in filtering foreign substances and even cancerous cells.

Structure

The lymph node is a kidney-shaped organ, ranging in size from 0.1cm to 2.5cm and is encapsulated by a fibrous capsule. The organ is divided into two main regions, namely outer cortex and inner medulla. Hilum is a structure found on the concave face of the lymph node form where the lymphatic vessels leave and blood vessels enter and leave.

Capsule

The capsule of the lymph node is composed of dense irregular connective tissue and collagenous fibres. The capsule extends trabeculae (a rod-like structure that anchors the framework of an organ) into the lymph node radiating towards the centre. The larger trabeculae are divided into finer bands and form a mesh-like network in the centre or medullary region of the lymph node. The spaces formed by the mesh contain the lymph node substance or actual lymphoid tissue.

Subcapsular Sinus

The space between the capsule and the cortex is known as subcapsular sinus. It is also known as lymph path, marginal sinus or lymph sinus that help in the transportation of lymphatic fluid. It receives the afferent vessels, goes on with the trabecular sinuses and connects with the medullary sinus in the medulla of the lymph node. The subcapsular sinus is present just below the capsule and consists of both reticular cells and fibres.

Cortex

The cortex is the outer layer of the lymph nodes that is present right below the subcapsular sinus. The cortex is divided into an outer cortex and an inner cortex, also called paracortex. The outer cortex, also called the B-cell layer, consists of B-cells arranged in follicles. The B-cells develop a germinal centre when elicited by an antigen. The paracortex consists mainly of T-cells and hence is also known as the T-cell layer.

Medulla

The innermost layer of the lymph node is the medulla that consists of medullary cords, sinuses and large blood vessels. The antibody-producing cells such as macrophages, plasma cells and B-cells are found in the medullary cords. The medullary sinuses are vessel shaped spaces that differentiate the medullary cords. Medullary sinuses are composed of histiocytes and reticular cells that obtain lymph from the cortical and trabecular sinuses. The lymph from the medullary sinus is drained into efferent lymphatic vessels.

Cells of the Lymph Node

The outer cortex is mainly full of B-cells arranged in follicles, while the inner cortex (paracortex) contains the T-cells and dendritic cells. The medulla has fewer cells in comparison to the cortex. It contains macrophages and plasma cells.

The B cell follicles have follicular dendritic cells and the T cell cortex has fibroblastic reticular cells. The reticular network gives structural support and helps in the adhesion of macrophages, dendritic cells and lymphocytes by providing a surface. It allows easy exchange of material with the blood that are necessary for the activation and maturation of immune cells.

Refer: Difference between T Cells and B Cells

Flow of Lymph

The lymph enters the lymph node through afferent lymphatic vessels on the convex side. The lymph then flows into the space of the subcapsular sinus and then continues into the sinuses of the cortex. From the cortex, the lymph collects into the medullary sinus. The lymph then exits the lymph node from the hilum on the concave side through the efferent lymphatic vessels.

Function of Lymph Nodes

The lymph node is a secondary lymphoid organ that functions primarily to filter the lymph and fight foreign substances. To fight these infectious agents, lymph contains lymphocytes such as B-cells and T-cells.

The B-cells that are produced in the bone marrow are transported to the lymph nodes and reside inside it. The B-cells circulate in the targeted area and stimulate immune response by producing antibodies if they find any infectious agents.

Antigens are chemical substances that are found in bacterial cell walls. Antigen-presenting cells such as dendritic cells take up these antigens and present it to the T-cells. If the T-cells have an appropriate receptor, they get activated and produce antibodies.

Clinical Significance

  • Swelling: The condition where the lymph nodes swell up is known as lymphadenopathy. The swelling in the lymph node can be indicative of drug reactions, autoimmune diseases, infection, tumours, lymphoma or leukaemia. The enlargement of the lymph node in a localised area can be suggestive of tumours which can be painful. More generalised swelling of the lymph nodes mostly indicates the risk of infection or autoimmune disease.
  • Lymphedema: It is the condition of swelling of lymph tissues due to incomplete drainage by the lymphatic system. Primary lymphedema is when the condition is congenital and either the lymph nodes are underdeveloped or totally absent. Secondary lymphedema is when the lymph nodes are removed such as in breast cancers or by radiation therapy. Lymphedema can also occur by parasitic infections.
  • Cancer: Lymph nodes can be affected by primary cancers of the lymph tissue or secondary cancers of other body parts. Primary cancers of the lymph are called lymphomas that include both Hodgkin and non-Hodgkin lymphomas. Most lymphomas are of B-cells that can be painless and slow growing or painful and undergoing sudden enlargement.

Secondary cancers of other body parts often metastasise to the lymph nodes and often the lymph nodes become the key part of cancer diagnosis for imaging and finding out the stage of cancer.

Stay tuned to BYJU’S Biology for more updates.

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Frequently Asked Questions

Q1

What is the shape of lymph nodes?

The lymph nodes are ovoid to bean-shaped.
Q2

What is lymph?

Lymph is a colourless liquid that is derived from blood plasma and is made up of lymphocytes.
Q3

What cells are found in the lymph nodes?

The lymph nodes primarily consist of B-cells and T-cells. It also contains other cells such as dendritic cells, macrophages and plasma cells.
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