The Lymphatic System

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Last updated 30 March 2006

This page was contributed by David Boshell.

The Lymphatic System

The lymphatic system is a specialised form of connective tissue consisting of cells, tissues and organs that react to the presence of potentially harmful (including foreign) antigenic substances.
It includes the thymus, spleen, lymph nodes, lymphatic nodules and diffuse lymphatic tissue, and may be collectively referred to as the immune system.

It is our 2^nd line of defence, after the epithelial covering of the body.

Primary Lymphatic Organs and Tissues

The primary (central) lymphatic organs and tissues include the thymus, bone marrow and gut associated lymphatic tissue (GALT).

They are sites of antigen-independent proliferation and differentiation into cells pre-programmed to recognise specific antigens.
These immunocompetent cells then enter the blood and lymph to be dispersed in the connective tissue and penetrate into the epithelia that line the mucosal surfaces.

Secondary Lymphatic Organs and Tissues

These are the effector lymphatic tissues.
They include the lymphatic nodules, lymph nodes, tonsils and the spleen.

Here lymphocytes undergo antigen-dependent proliferation and differentiation into effector lymphocytes and memory cells.

Lymphocytes

These are immunocompetent cells that have the ability to:

Distinguish between molecules of an organism and foreign bodies;
Inactivating or destroying foreign organisms or toxins, thus providing the protective response known as immunity.

There are small, medium and large lymphocytes.
The medium and mostly small size lymphocytes are found in circulation.

Larger, activated lymphocytes may be found in lymph nodes.

Lymphocytes originate from stem cells in bone marrow.
They differentiate, by different routes, into two functional types: T Lymphocytes (T Cells) and B Lymphocytes (B Cells).

B Cells, T Cells and the Response to Antigens

The immune system responds to antigens in 2 ways:

A cell mediated response;
And an antibody-mediated or humoral response.

B Lymphocytes

These evolve in bone marrow and GALT, and are part of the humoral response.
They will only react with the antigen they have been genetically programmed for.
Once activated by this antigen, they may differentiate and proliferate into either:

Plasma cells, that produce antibodies.
The antibody binds, forming an antibody-antigen complex, that may be phagocytosed by macrophages.
It may also activate a complement system of proteins that bind to foreign cells for lysing or phagocytosis.

Memory cells, which, after exposure to the specific antigen, will be able to participate in a rapid, secondary response with the same antigen.
They do not participate in an initial or primary response.

T Lymphocytes

These evolve in the thymus and are part of the cell-mediated or thymus-dependent response to antigens.
Upon interaction with an antigen, they will differentiate and proliferate into memory cells, and 3 types of effector T lymphocytes:

Cytotoxic lymphocytes (killer T cells), the primary effector cells in cell-mediated immunity.
These cells scan the surface of other cells for signs of viral infection or abnormality, killing them if necessary by causing them to lyse.

Helper T lymphocytes, which recognise foreign antigens presented by macrophages.
After this, they release interleukin hormones to stimulate ‘processed’ B cells to produce antibodies.

Suppresser T lymphocytes, that suppress the activity of B cells.

Lymphokines and Interleukins

Lymphokines are substances release by lymphocytes on contact with a specific antigen.
These stimulate the activity of monocytes and macrophages in the cell-mediated immune response.

Macrophages

These are involved in both types of immune response.
They can process and present the antigen to the B cells or helper T cells.
Or they can destroy the antigen by digestion after it has been processed by other cells of the immune system.

Lymphatic Vessels and Lymphocyte Circulation

Lymphatic vessels begin as a network of capillaries in the loose connective tissue, most numerous under the epithelium of the skin and mucous membranes.

Lymphatic capillary walls are more permeable than those of blood capillaries, and thus more readily drain substances from the extracellular spaces of connective tissue.

Lymph vessels pass through lymph nodes in the lymphatic circulation.
Here antigens in the lymph are concentrated by dendritic cells and presented to lymphocytes, leading to the immune response.

These vessels ultimately drain into the thoracic duct, and thence to the bloodstream at the junction of the internal jugular and subclavian veins in the neck.

Lymphocytes reach sites of the body via the lymphatic circulation and enter the lymph nodes.

B cells will migrate to the medulla and the lymphatic nodules of the cortex.
T cells migrate to the paracortex.

Lymphocytes are conveyed to and from the lymphatic tissues via the blood vessels.
They recirculate in all tissues except the thymus.

Diffuse Lymphatic Tissue

The alimentary canal, respiratory passages and genitourinary tract are guarded by accumulations of diffuse lymphatic tissue, which are not encapsulated.
Cells of this tissue are found in the lamina propria of these tracts.

They are strategically placed to intercept the entry of antigens, travel to regional lymph nodes, and undergo proliferation and differentiation, with effector lymphocytes, plasma cells and memory cells returning.
Large numbers of plasma cells and eosinophils can be found in the lamina propria of the GIT.

Lymphatic Nodules (follicles) and GALT

These are random, localised concentrations of lymphocytes.
They are sharply defined, but not encapsulated, and are found mainly in the walls of the alimentary canal, respiratory passages and genitourinary tract.

There are notable aggregations of these nodules in the alimentary canal, including:

The tonsillar ring (of Waldeyer) in the oropharynx, including the pharyngeal, palatine and lingual tonsils;
Peyer’s patches in the ileum;
Aggregations in the caecum and appendix.

The diffuse lymphatic tissues and their aggregations of the alimentary canal form the gut associated lymphatic tissue (GALT).
Lymphatic nodules are also the basic structural unit of the lymph node.

Features of a lymphatic nodule include:

A germinal centre. This region develops when a lymphocyte has recognised an antigen, and is an indicator of lymphatic tissue response to antigen. It stains less intensely than;
The marginal zone containing smaller lymphocytes.

Lymph Nodes

These are small, encapsulated organs located along the pathway of lymphatic vessels.
They range in size from 1mm - 2cm.

They are filters for lymph on its way to the blood and phagocytose any particulate matter.
Lymph nodes are concentrated in regions such as the axillae, groin, mesenteries, neck, cubital and popliteal fossae.

Structure of Lymph Nodes

There is a dense connective tissue capsule surrounding the node.
Trabeculae, invade the substance of the node from the capsule.
Reticular tissue, composed of reticular cells and their fibres, forms a supporting meshwork throughout the organ.

Afferent lymph vessels bring lymph to the node, entering at various points on the convex surface of the capsule.

A single efferent lymph vessel conveys lymph away from the node.
It leaves at the hilum, a depression on the concave surface of the node and gateway for blood vessels and nerves.

The parenchyma of the lymph node consists of:

The core, medulla. It consists of cords of lymphatic tissue separated by lymphatic or medullary sinuses.
These sinuses converge toward the hilum, draining into the efferent lymphatic vessel.

The outer cortex, contain many reticular fibres, lymphocytes, macrophages, plasma cells, and lymphatic sinuses.
Lymphocytes in the outer part of the cortex are organised into nodules.
The paracortical zone or deep cortex, adjacent to the medulla, is free of nodules.
Development of this region depends on the supply of T cells, and is thus also known as the thymus-dependent cortex.

B lymphocytes are mainly in the cortex and medullary cords.

Just under the capsule of the node, there is a subcapsular sinus.
This is for the drainage of afferent vessels.

Trabecular sinuses extend through the cortex with the trabeculae, draining into the medullary sinuses.
Within these sinuses, macrophage and reticular cell processes span the lumen of the sinus, forming a criss-crossing meshwork, retarding and filtering the lymph, and trapping any materials for phagocytosis.

Most lymphocytes enter the node via postcapillary venules in the deep cortex.
The venules are lined by cuboidal or columnar endothelial cells.
It allows lymphocytes to cross the endothelium from the bloodstream, but prevents the passage of fluid into it.

Lymphocytes may leave the node via the sinuses, and the efferent lymphatic vessel.

The Thymus

The thymus is a bi-lobed organ in the superior mediastinum, anterior to the heart and great vessels.
It develops bilaterally, out of epithelial invaginations from the third oropharyngeal pouch.
The thymus is where stem lymphocytes proliferate and differentiate into T lymphocytes.

The function of the thymus is as the site of maturation of T cells.

The parenchyma of the thymus consists of epithelioreticular cells.
These are epithelial cells that have a stellate shape and form a cytoplasmic reticulum: a framework for the thymic lymphocytes.
These epithelioreticular cells correspond to the other reticular cells in lymphatic tissues, but there are no reticular fibres in the thymus.

Structure of the Thymus

An outer cortex, densely populated with lymphocytes, thus very basophilic.
Also, there are many macrophages present for the phagocytosis of 80% of lymphocytes which are self reacting (programmed for their own antigens).

An inner medulla that stains less intensely, containing larger lymphocytes with more cytoplasm.
Epithelioreticular cells can also be seen here.
A distinctive feature of the thymic medulla are thymic or Hassall’s corpuscles, masses of concentrically arranged epithelioreticular cells, that may be partly keratinised.

An external capsule of connective tissue which extends trabeculae into the margin of the cortex and medulla.
It contains blood vessels, efferent lymphatic vessels, and nerves.
The trabeculae create thymic lobules, which are just cortical caps over the medullary tissue.

Changes of Thymic Structure with Age

The thymus is fully functional at 20 weeks of foetal life.
Its features change over time, however, with the progressive involution of adipose tissue.
Starting in the juvenile, there is isolation of cortical compartments, reduction of cortical and medullary volume, and the appearance of more, larger blood vessels, until the adult thymus is mainly dominated by fat.

The Blood-Thymic Barrier

Blood vessels enter the substance of the thymus from the trabeculae, carrying a perivascular connective tissue sheath with them.
This sheath is covered by a basal lamina and another sheath of epithelioreticular cells.

This blood-thymic barrier prevents molecules from passing to the tissue spaces.
The layers of this barrier, from the lumen outwards, are:

Capillary endothelium
Endothelial basal lamina
A thin perivascular connective tissue sheath containing many macrophages
Basal lamina of the epithelioreticular cells
Epithelioreticular cell sheath

The Spleen

The spleen is the largest lymphatic organ, located in the upper left quadrant of the abdominal cavity.

The spleen functions to filter the blood, and react immunologically to blood-borne antigens.

Functions of the Spleen

Immune functions include the proliferation of B and T lymphocytes.
Also, there is the production of humoral antibodies and the removal of macromolecular antibodies from blood.

Haemopoetic functions include the formation and removal of blood cells and platelets.
Also, there is the retrieval of iron from red cell haemoglobin, and the storage of blood in some species.

The spleen, however, is not essential.

Structure of the Spleen

There is an external capsule of dense connective tissue.
From here, trabeculae extend into the substance of the organ.

This connective tissue also contains myofibroblasts, and is thus contractile.

On the medial surface of the spleen, the hilum allows passage of the splenic vessels, nerves and lymphatic vessels.

The substance of the spleen is known as the splenic pulp.
This pulp is divided into white pulp areas, surrounded by red pulp.

The White Pulp

This mainly consists of lymphocytes.
Branches of the splenic artery course through the trabeculae and then enter the white pulp, known as the central artery in this region.
The lymphocytes aggregated around the central artery in a cylindrical fashion constitutes the periarterial lymphatic sheath (PALS) of the artery.
Lymph nodules in the PALS may displace the central artery from its central position in the white pulp.

The Red Pulp

This has large numbers of red blood cells (RBCs).
It consists of splenic sinuses, separated by splenic cords (of Billroth).

The splenic cords are a loose meshwork of reticular cells and fibres, containing blood cells and immunological cells.
Iron is recycled from the phagocytosis of RBCs.

The venous splenic sinuses are special sinuses, lined by extremely long endothelial cells, longitudinally arranged along the vessel, with spaces between them for the passage of blood cells.

Splenic Circulation

The splenic artery branches into the trabecular, the central arteries.
The central arteries of the white pulp then branch into penicillar arterioles in the red pulp.
These are actually capillaries, and may be sheathed by aggregations of macrophages.
Blood from these penicilli leaves the vascular system to populate the splenic pulp, before re-entering the red pulp.

Two theories of circulation through the splenic arterioles exist.

Closed Circulation Model

In this model, the splenic arterioles are a "continuous vascular channel".
They only empty into the splenic sinuses of the red pulp.
The blood then leaves the sinuses before re-entering them.

Open Circulation Model

In this model, the arterioles empty directly into the splenic cords.
Thus, blood percolates through the reticular meshwork of the pulp.
It then only enters the splenic sinuses from the extravascular side.

This model has more supporting evidence than the former.