Lymphatic System-component, Lymphatic follicle, Lymph Nodes, Haemal nodes, Macrophage System, Lymphoid tissue .

Introduction to the Lymphatic System .

Lymphatic System

1. Lymphatic system is essentially a drainage system which is accessory to the venous system . Most of the tissue fluid formed at the arterial end of capillaries is absorbed back into the blood by the venous ends of the capillaries and the postcapillary venules. 

2. The rest of the tissue fluid (10-20%) is absorbed by the lymphatics which begin blindly in the tissue spaces. 

3. It is important to know that the larger particles (proteins and particulate matter) can be removed from the tissue fluid only by the lymphatics. Therefore, the lymphatic system may be regarded as ‘drainage system of coarse type’ and the venous system as ‘drainage system of fine type’. 

4. Certain parts of the lymphatic system (lympho-reticular organs), however, are chiefly involved in phagocytosis, raising immune responses and contributing to cell populations of the blood and lymph. 

5. The tissue fluid flowing in the lymphatics is called lymph. It passes through filters (lymph nodes) placed in the course of lymphatics, and finally drains into the venous blood. 

6. Lymph from most of the tissues is clear and colourless, but the lymph from small intestine is milky-white due to absorption of fat. The intestinal milky lymph is called chyle and lymph vessels, the lacteals. 

Difference between Lymph capillaries & Blood Capillaries . 

Difference between lymph capillaries & blood Capillaries .

 

Components of Lymphatic System .

 The lymphatic system comprises: 

 (1) lymph vessels.

 (2) central lymphoid tissues.

 (3) peripheral lymphoid organs .

 (4) circulating lymphocytes. 

1. Lymph Vessels .

Relationship between Lymphatic & circulatory System

[1]. The lymph capillaries begin blindly in the tissue spaces  and form intricate networks. Their calibre is greater and less regular than that of blood capillaries, and their endothelial wall is permeable to substances of much greater molecular size . 

[2]. Lymph capillaries are absent from the cellular structures like brain, spinal cord, splenic pulp, and bone marrow. The lymph capillaries join to form lymphatics, which are superficial and deep lymphatics. The superficial lymphatics accompany veins, while the deep lymphatics accompany arteries. 

[3]. The lymph passes through filters or barriers of the regional lymph nodes which trap the particulate matter. The filtered lymph passes through larger lymphatics and is eventually collected into two large trunks, the thoracic duct and right lymphatic duct, which pour their lymph into the brachiocephalic veins . 

[4]..Thoracic duct drains both lower limbs, abdomen, left halves of thorax, head and neck and left upper limb. Right lymphatic duct drains right halves of thorax, head and neck and right upper limb. The lymphatics anastomose freely with their neighbours of the same side as well as of the opposite side.

[5].Larger lymphatics are supplied with their vasa vasorum and are accompanied by a plexus of fine blood vessels which form red streaks seen in lymphangitis. 

2. Central Lymphoid Tissues .

[1]. Central lymphoid tissues comprise bone marrow and thymus. All ‘pluripotent’ lymphoid stem cells are initially produced by bone marrow, except during early fetal life when these are produced by liver and spleen. 

[2]. The stem cells undergo differentiation in the central lymphoid tissues, so that the lymphocytes become competent defensive elements of the immune system. Bone marrow helps differentiation of the (committed B-lymphocytes which are capable of synthesizing antibodies after getting transformed into plasma cells. 

[3]. In birds, B-cells are differentiated in the wall of the bursa of Fabricius a hindgut diverticulum. Thymus helps differentiation of immunologically competent but uncommitted T-lymphocytes (10% of thymic population) which are long lived, join the circulating pool of lymphocytes and populate the thymus dependent areas of lymph nodes and other peripheral lymphoid organs. 

[4]. T-cells being uncommitted can react to a wide range of foreign antigenic stimuli. These respond by cytotoxic cell killing (killing virus-infected cells, neoplastic cells, fungi, tissue grafts, etc.), by ‘arming’ macrophages, and by triggering the large mononuclear cells (killer cells) and the ‘helper’ activity of B-lymphocytes. 

3. Peripheral Lymphoid Organs .

[1]. Peripheral lymphoid organs comprise lymph nodes, spleen, and epithelio-lymphoid tissues (lymphoid nodules developed in the alimentary and respiratory tracts). Any part of this may become overactive on appropriate stimulation. 

[2]. The progenies of B- and T-lymphocytes reach these organs where the cells may proliferate and mature into competent cells. The mature lymphocytes join the circulating pool of lymphocytes. 

4. Circulating Pool of Lymphocytes.

The pool contains mature progenies of B- and T-lymphocytes which may be called upon during antigenic emergencies (Roitt, 1977). 

Lymphatic Follicle (Nodule) .

 Collections of lymphocytes occur at many places in the body. Everywhere there is a basic pattern, the lymphatic follicle. The follicle is a spherical collection of lymphocytes with a pale Centre known as germinal Centre, where the lymphocytes are more loosely packed. The central cells are larger in size, stain less deeply, and divide more rapidly, than the peripheral cells. 

Lymph Nodes .

Lymph nodes of neck

[1]. Lymph nodes are small nodules of lymphoid tissue found in the course of smaller lymphatics. The lymph passes through one or more lymph nodes before reaching the larger lymph trunks. 

[2]. The nodes are oval or reniform in shape, 1-25 mm long, and light brown, black (pulmonary) or creamy white (intestinal) in colour. Usually they occur in groups (cervical, axillary, inguinal, mesenteric, mediastinal, etc.), but at times there may be a solitary lymph node. 

[3]. Superficial nodes are arranged along the veins, and the deep nodes along the arteries. 

[4]. Cervical lymph nodes form a ring at the junction of head and neck and vertical chains in the neck . These drain whole of head and neck. 

[5]. On right side lymph vessels drain into right lymphatic duct, while on left side these drain into thoracic duct. Lymph vessels of abdominal wall above a line passing horizontally through umbilicus drain into respective sides of axillary lymph nodes. 

[6]. Lymph vessels below this line drain into inguinal group of lymph nodes. This line is called “watershed” .Each lymph node has a slight depression on one side, called hilum. The artery enters the node and the vein with efferent lymphatic comes out of it, at the hilum. The afferent lymphatics enter the node at different parts of its periphery. 

Structurally, a lymph node is made up of the following parts . 

Structure of lymph node

1. Fibrous and reticular framework: 

The lymph node is covered by a capsule. From the deep surface of the capsule a number of trabeculae extend radially into the interior of the node, where they are continuous with the fine reticulum which forms the supporting framework for the lymphoid tissue. 

2. Lymphatic channels: 

The subcapsular sinus lies beneath the capsule and surrounds the node except at the hilum. Many afferent lymphatics of the node open into the subcapsular sinus. Lymph filters through reticulin fibres and leaves the node by one efferent lymphatic vessel. 

3. Cortex: 

Zones in lymph node.  AL = afferent lymphatics; EL = efferent lymphatic

It is the outer part of the lymph node situated beneath the subcapsular sinus, being absent at the hilum. It is made up of lymphatic follicles and is traversed by fibrous trabeculae. The cortex is far more densely cellular than the medulla. 

It is divided into: 

(a) Zone 1 containing loosely packed small lymphocytes, macrophages and occasional plasma cells in the periphery of the follicle and extending into the medullary cords. 

(b) Zone 2 containing more densely packed small lymphocytes and macrophages, deep to zone I and limited to cortical and paracortical (inner cortex) areas. 

(c) Zone 3 including the germinal centre which contains large lymphocytes and macrophages. The maturing lymphocytes pass from zone 3 to zone 2 to zone 1 and to the lymph sinus . 

According to the distribution of B- and T-lymphocytes, the cortex is divided into: 

1. An outer part which contains immature B-lymphocytes. 

2. An inner part, between the germinal centre and the medulla, which contains T-lymphocytes. This part is known as paracortex or thymus dependent zone. The mature B-lymphocytes (plasma cells) are found in the medulla. 

4. Medulla: 

It is the central part of the lymph node, containing loosely packed lymphocytes (forming irregular branching medullary cords), the plasma cells, and macrophages. 

5. Blood channels: 

The artery enters at the hilum and divides into straight branches which run in the trabeculae. In the cortex the arteries further divide to form arcades of arterioles and capillaries with many anastomosing loops. The capillaries give rise to venules and veins, which run back to the hilum. The capillaries are more profuse around the follicles, and the postcapillary venules are more abundant in the paracortical zones for lymphatic migration. 

Haemal Nodes .

These are small lymphatic bodies resembling lymph nodes in their structure, which are found in the course of blood vessels. The afferent and efferent lymphatics are absent. Their sinuses are filled with blood rather than lymph. These are found in some animals in relation to their abdominal and thoracic viscera. Haemal nodes may represent an intermediate stage between a lymph node and the spleen. In man, the spleen is a large haemal node. 

Mononuclear Phagocyte System or Macrophage System (Reticulo-endothelial System) .

 This system is closely related to lymphatic system because the two are independent structurally and functionally. The macrophage system is made up of highly phagocytic cells which are widely distributed in the body. 

These cells include: (a) Macrophages of connective tissue, reticular tissue and lungs .

(b) Monocytes of blood .

(c) Kupffer’s cells of liver .

(d) Meningocytes of meninges .

(e) Microglial cells of nervous tissue  .

(f) Foreign body giant cells.  The endothelial cells, fibroblasts, and most leucocytes are not included in this system because of their poor power of phagocytosis. 

Functions of  Macrophage System .

1. The system forms first line of defence of the body against microorganisms, because of the amoeboid and phagocytic properties of its cells. 

2. The macrophages of lymphoid tissue are now considered to be intimately concerned with mounting specific immune responses by the neighbouring cells. 

3. Many of the prominent sites of RES are also important sites of haemopoiesis. 

Growth Pattern of Lymphoid Tissue .

 Lymphoid tissue of the body is prominent at birth and grows rapidly during childhood. The growth ceases at about the time of puberty and is followed by partial atrophy in the later years. This growth pattern is shared by lymph nodes, thymus, tonsils, lymphoid tissue of the intestines, and the follicles of spleen. 

However, the lymph nodes may enlarge again in response to inflammation (lymphadenitis) or tumor formation (Hodgkin’s disease, lymphosarcoma, etc.). Lymph nodes are commonly enlarged by metastasis (spread) of the malignant growths (carcinoma). 

Functions of Lymphoid System  .

1. Lymph capillaries absorb and remove the large protein molecules and other particulate matter from the tissue spaces. Thus the cellular debris and foreign particles (dust particles inhaled into the lungs, bacteria and other microorganisms) are conveyed to the regional lymph nodes. Lymphatics (lacteals) help in transportation of fat from the gut. 

2. Lymph nodes serve a number of functions. 

(a) They act as filters for the lymph which percolates slowly through the intricate network of its spaces. Thus the foreign particles are prevented from entering the bloodstream. 

(b) The foreign particles are engulfed by the macrophages in the sinuses. 

(c) Antigens are also trapped by the phagocytes. 

(d) The mature B-lymphocytes (plasma cells capable of producing antibodies) and mature T-lymphocytes are produced in the node. 

(e) Both the cellular and humoral immune responses are mounted against the antigen-laden phagocytes.

(f) The circulating lymphocytes can pass back into the lymphatic channels within the node,

 (g) Humoral antibodies are freely produced by the lymph nodes. 

3. Production (proliferation) and maturation of B- and T-lymphocytes is the main function of lymphoid tissue. 

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