The lymphoid organs’ function is to administer immunity because that the body. This second article in a six-part collection explains the major and an additional lymphoid organs and also their clinical significance and structure. It comes v a self-assessment enabling you to check your understanding after reading it


This post is the second in a six-part series about the lymphatic system. It discusses the role of the lymphoid organs, i m sorry is to construct and carry out immunity for the body. The main lymphoid organs room the red bone marrow, in i beg your pardon blood and also immune cells space produced, and the thymus, whereby T-lymphocytes mature. The lymph nodes and spleen are the significant secondary lymphoid organs; castle filter the end pathogens and also maintain the population of tires lymphocytes.

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Citation: Nigam Y, article J (2020) The lymphatic system 2: framework and duty of the lymphoid organs. Nursing time ; 116: 11, 44-48.

Authors: Yamni Nigam is professor in biomedical science; john Knight is combine professor in biomedical science; both in ~ the university of Human and Health Sciences, Swansea University.

This short article has been double-blind peer reviewed


This write-up discusses the significant lymphoid organs and their role in developing and also providing immunity because that the body. The lymphoid organs incorporate the red bone marrow, thymus, spleen and also clusters that lymph nodes (Fig 1). Castle have many functional duties in the body, most notably:

Production that blood cells, consisting of red blood cells (erythrocytes), white blood cells (leucocytes) and also platelets (thrombocytes);Removal of damaged red blood cells;Maturation of immune cells;Trapping foreign material.

The red bone marrow and also thymus are taken into consideration to be major lymphoid organs, due to the fact that the majority of immune cell originate in them.


The red bone marrow

Bone marrow is a soft, gelatinous tissue present in the central cavity of long bones such together the femur and humerus. Blood cells and immune cells arise native the bone marrow; they develop from immature stem cell (haemocytoblasts), which follow unique developmental pathways to come to be either erythrocytes, leucocytes or platelets. Stem cells rapidly multiply to do billions the blood cells every day; this process is recognized as haematopoiesis and is outlined in Fig 2.


To ensure there is a continuous production and differentiation of blood cell to change those lost to role or age, haematopoietic stem cell are present through adulthood. In the embryo, blood cell are at first made in the yolk sac but, as advance of the embryo proceeds, this duty is taken over by the spleen, lymph nodes and liver. Later in gestation, the bone marrow take away over most haematopoietic functions so that, at birth, the whole skeleton is filled through red bone marrow.

Red bone marrow produces all erythrocytes, leucocytes and platelets. Haematopoietic stem cells in the bone marrow follow one of two people the myeloid or lymphoid lineages to produce distinct blood cells (Fig 2); these incorporate myeloid progenitor cell (monocytes, macrophages, neutrophils, basophils, eosinophils, erythrocytes, dendritic cells and platelets), and also lymphoid progenitor cells (T-lymphocytes, B-lymphocytes and also natural killer cells).

Some lymphoid cells (lymphocytes) begin life in the red bone marrow and become totally formed in the lymphatic organs, consisting of the thymus, spleen and also lymph nodes. Together puberty is reached and also growth slow down, physiological conversion occurs, changing red bone marrow come yellow bone marrow. This entire process is completed by the age of 25 years, when red bone marrow distribution shows its adult sample in the bones.

The pattern is characterised by:

The existence of red bone marrow in the axial skeleton (the vertebral bodies, sacral bone and also medial components of the i know good bones) and articular end of the humeral and femoral bones;The many distal components of the skeleton being filled through yellow bone marrow only, which mainly acts as a keep for fats, providing sustenance and also maintaining the correct atmosphere for the bone come function.

However, under details conditions, such as severe blood ns or fever, the yellow marrow might revert ago to red marrow (Malkiewicz and Dziedzic 2012).

Bone marrow diseases and transplants

Any condition or disorder that poses a risk to the bone marrow can influence many body systems, particularly if it stays clear of stem cells from transforming into essential cells. Those well-known to damages the marrow’s productive capacity and damage stem cell include:

Leukaemia;Hodgkin’s lymphoma;Other lymphomas.

A growing variety of diseases can be treated v a bone marrow transplant or haematopoietic stem cabinet transfer; this is often accomplished by harvesting an ideal donor stem cells from the posterior iliac crests that the i know good bone, where the concentration of red bone marrow is highest.

The thymus

The thymus gland is a bi-lobed, pinkish-grey organ located just over the heart in the mediastinum, wherein it rests listed below the sternum (breastbone). Structurally, the thymus resembles a little bow tie, which gradually atrophies (shrinks) through age. In pre-pubescents, the thymus is a relatively huge and very active body organ that, typically, weighs approximately 40g, but in a middle-aged adult it may have actually shrunk sufficiently to be an overwhelming to locate. By twenty years of age, the thymus is 50% smaller than it to be at birth, and also by 60 years of period it has actually shrunk come a 6th of its initial size (Bilder, 2016); this is referred to as thymic involution

Each that the two lobes that the thymus is surrounded by a capsule, within which are numerous little lobules – generally measuring 2-3mm in broad – which are hosted together by loose connective tissue. Each lobule consists of follicles that are composed that a frame of thyomsin-secreting epithelial cells and also a populace of T-lymphocytes; this cells are typically referred to as T-cells (the ‘T’ denotes their origin as mature cell from the thymus). Lobules have two unique areas:

A dense outer cortex the is rich in proactively dividing T-cells;An inner medulla, i m sorry is lot paler in colour and functions as an area of T-cell maturation.

In addition to gift a major lymphoid organ, the thymus is also recognised as part of the endocrine system since it secretes a family of hormones jointly referred to as thymosin; this is a team of numerous structurally associated hormones secreted by the thymic epithelial cells. These hormones are crucial for common immune function and numerous members of the thymosin family members are used therapeutically to treat cancers, infections and diseases such as multiple sclerosis (Severa et al, 2019).

Role in T-cell maturation

T-cells originate together haematopoietic stem cell from the red bone marrow (Fig 2). A populace of this haematopoietic stem cells infiltrate the thymus, splitting further within the cortical areas of the lobules then migrating into the medullary regions to maturation into energetic T-cells; this process of T-cell mature is managed by the hormone thymosin. A ratio of these mature T-cells continually move from the thymus into the blood and also other lymphoid guts (spleen and lymph nodes), where they beat a significant role in the body’s certain immune responses (which will certainly be discussed in detail in component 3 of this series). The importance of this cells is apparent in patient who have depleted T-cell populations, such together those infected v HIV.

One that the most necessary functions of the thymus is programming T-cells to recognise ‘self’ antigens with a procedure called thymic education. This procedure allows maturation T-cells to identify foreign, and also therefore perhaps pathogenic, material from antigens the belong come the body. It has actually been demonstrated the removal of the thymus might lead to boost in autoimmune diseases, together this ability to recognise self is diminished (Sherer et al, 1999).

Clinical significance

Diseases of the thymus include thymic cancer and myasthenia gravis (MG). MG occurs as soon as the thymus produce antibodies the block or damage the muscle-receptor sites, leading to the muscle to end up being weak and easily tired. It most generally affects muscle that manage the eyes and also eyelids, leading to droopy eyelids and challenge making facial expressions; chewing, swallowing and also speaking likewise become difficult. MG can impact people of any kind of age, however typically beginning in ladies aged 60 years.

In most cases of one of two people MG or thymic cancer, thymectomy is recommended. Patients who have had actually a thymectomy may develop an immunodeficiency recognized as an excellent syndrome, which increases their susceptibility to bacterial, fungal and also viral opportunistic pathogens; this problem is, however, reasonably rare.

The spleen

The spleen and lymph nodes space two significant secondary lymphoid organs the play crucial roles in:

Filtering out and also destroying undesirable pathogens;Maintaining the populace of maturation lymphocytes (which room white blood cells) to permit the adaptive immune response to begin.

When international antigens reach these organs, they initiate lymphocyte activation and also subsequent clonal expansion and also maturation that these essential white blood cells. Maturation lymphocytes have the right to then leaving the second organs to enter the circulation, or travel to other areas, and target foreign antigens.

The spleen is the largest lymphoid organ. Situated in the upper left hypochondriac an ar of the abdominal muscle cavity, in between the diaphragm and also the fundus of the stomach, that primarily attributes as a filter because that the blood, pass it into close call with scavenging phagocytes (white blood cell in the spleen the will look for out and ‘eat’ any kind of pathogens in the blood) and lymphocytes.

Due to its comprehensive vascularisation, the spleen is a dark-purplish oval-shaped organ; in adult it is about 12cm long, 7cm wide and weighs roughly 150g. However, the size of the spleen deserve to vary v circumstance: the diminishes in starvation, after heavy exercise and following severe haemorrhage (Gujar et al, 2017), and recent investigate indicate boost in size in well-fed individuals and during the ingestion of food (Garnitschnig et al, 2020).

The spleen (Fig 3) is attached in a dense, fibro-elastic capsule that protrudes right into the body organ as trabeculae; these trabeculae constitute the organ’s framework. Blood enters the spleen indigenous the splenic artery and leaves via the splenic vein, both the which are at the hilum; the splenic vein ultimately becomes a tributary the the hepatic portal vein.


The spleen is comprised of two regions:

Stroma – making up the thick outer capsule with its trabeculae, some fibres and fibroblasts (cells the secrete connective tissue collagen);Parenchyma – created of two species of nested tissue called white pulp and also red pulp.

White pulp is a massive of germinal centres of separating B-lymphocytes (B-cells), surrounded by T-cells and also accessory cells, consisting of macrophages and dendritic cells; these cells are arranged as lymphatic nodules roughly branches of the splenic artery. As blood flows into the spleen via the splenic artery, it enters smaller, main arteries that the white pulp, ultimately reaching the red pulp. The red pulp is a spongy tissue, accounting for 75% that the splenic volume (Pivkin et al, 2016); it is composed of blood-filled venous sinuses and splenic cords.

Splenic cords are made up of red and also white blood cells and also plasma cells (antibody-producing B-cells); therefore, the red pulp primarily attributes as a filtration system for the blood, conversely, the white pulp is wherein adaptive T- and also B-cell responses are mounted. The colour of the white pulp is obtained from the closely packed lymphocytes and also the red pulp’s color is due to high number of erythrocytes (Stewart and McKenzie, 2002).


The spleen has actually three major functions:

To mountain an immune response and remove micro-organisms from circulation;To destroy damaged and also worn-out red blood cells;To store platelets (and blood).

The spleen’s main immunological duty is to remove micro-organisms from circulation. The lymphatic nodules space arranged together sleeves approximately the blood vessels, happen blood right into the spleen. Within the white pulp space splenic nodules dubbed Malpighian corpuscles, which are rich in B-cells, so this section of lymphoid organization is quick to answer to international antigenic stimulation by producing antibodies. The wall surfaces of the meshwork of sinuses in the red pulp also contain phagocytes the engulf international particles and cell debris, successfully filtering and also removing them from circulation.

In the spleen’s devastation of old and senescent red blood cells, they are digested by phagocytic macrophages in the red pulp. The haemoglobin is then split apart right into haem and globin. The globin is broken down right into its constituent amino acids, which have the right to be utilised in the synthetic of a brand-new protein. Haem is composed of an stole atom surrounded 4 non-iron (pyrrole) rings.

The steel is removed and transported to it is in stored as ferritin, then reused come make brand-new haemoglobin in the red bone marrow; macrophages transform the pyrrole rings into the green pigment biliverdin and also then into the yellow colours bilirubin. Both space transported to the liver bound to plasma albumin. Bilirubin, the more toxic pigment, is conjugated in the liver to type a less toxic compound, i beg your pardon is excreted in bile.

The red pulp partially serves to save a huge reserve the the body’s platelets – approximately a 3rd of the full platelet supply. In some pets – specifically athletic mammals such together horses, greyhounds and foxes – the spleen is also critical reservoir that blood, i beg your pardon is released into circulation during times of tension to improve aerobic performance. In humans, however, the spleen contributes just a tiny percentage of blood cells into energetic circulation under physiological stress; the full stored blood volume is believed to be only 200-250ml (Bakovic et al, 2005). The capsule that the spleen might contract complying with haemorrhage, release this reserve into circulation in the body.

The spleen also plays a minor role in haematopoiesis: typically occuring in foetuses of increase to five months’ gestation, erythrocytes, in addition to the bone marrow, are produced by the spleen.

Clinical significance

As the spleen is the largest collection the lymphoid organization in the body, epidemic that cause white blood cabinet proliferation and also antigenic stimulation may cause germinal centres in the body organ to expand, resulting in its enlargement (splenomegaly). This wake up in numerous diseases – because that example, malaria, cirrhosis and also leukaemia. The spleen is not normally palpable, yet an enlarged spleen is palpable throughout deep inspiration. Enlargement may also be led to by any obstruction in blood flow, for example in the hepatic portal vein.


The anatomical place of the spleen coincides with the left tenth rib. Offered its proximity to the ab wall, it is one of the most frequently injured organs in blunt abdominal trauma. The spleen is a delicate organ and, because of its extremely vascularised nature, any kind of injury resulting in rupture will certainly rapidly result in severe intraperitoneal haemorrhage; death may an outcome due to substantial blood loss and shock.

A center splenic injury may be controlled conservatively, but an generally burst or ruptured spleen might be treated by complete and prompt removed (splenectomy). However, present data supports successful non-operative administration of numerous traumatic splenic injuries, through the intention of to reduce the need for complete removal (Armstrong et al, 2019).

Patients gift treated for particular malignant conditions may additionally require a partial or full splenectomy and, although other structures such as the bone marrow and also liver deserve to take over several of the features that room usually carried out through the spleen, such patients might be at boosted risk the infection. Through an overwhelming post-splenectomy infection, there is additionally an raised risk the sepsis, which is connected with far-ranging morbidity and mortality. Epidemic is usually with encapsulated pathogens, including Streptococcus pneumoniae, Haemophilus influenzae and also Neisseria meningitidis. Clinical guidelines to aid reduce the risk of infection support education around infection prevention, vaccination and also antibiotic prophylaxis (Arnott et al, 2018).

“Swollen lymph nodes and also a fever are sure signs that the body is mounting an effective immune response versus an offending pathogen”

Lymph nodes

Lymph nodes differ in size and shape, yet are commonly bean-shaped structures uncovered clustered at particular locations throughout the body. Back their dimension varies, every node has actually a characteristic inner structure (Fig 4).


The main portions the the lymph node are important to that is function; here, over there are big numbers of resolved macrophages, i beg your pardon phagocytose international material such as bacteria top top contact, and populations of B- and T-cells. Lymph nodes are an essential to many antibody-mediated immune responses: when the phagocytic macrophages trap pathogenic material, that product is presented to the lymphocytes for this reason antibodies can be generated.

Each lymph node is supplied by one or an ext afferent lymphatic vessels, which provide crude, unmodified lymph directly from neighbouring tissues. A healthy, totally functioning node clears the bulk of pathogens native the lymph prior to the liquid leaves via one or an ext efferent lymphatic vessels. In enhancement to the lymphatic supply, every lymph node is offered with blood via a little artery; the artery delivers a range of leucocytes, i beg your pardon populate the inner areas of the node.

When epidemic is present, the lymph nodes become increasingly metabolically active and their oxygen requirements increase. A little vein dead deoxygenated blood away from every node and returns it come the major veins. In time of infection, this venous blood may carry a range of chemistry messengers (cytokines) the are created by the resides leucocytes in the nodes. These cytokines action as basic warning signals, alerting the body to the potential threat and activating a selection of particular immune reactions.


The structure of a lymph node is not unlike that of the spleen. Each lymph node is split into number of regions:

Fibrous capsule – this creates a protective outer sheath and also has trabeculae that prolong periodically right into the node, subdividing it into tiny compartments;Outer cortex (nodular cortex) – just inside the capsular margin, this consists of countless follicles that room rich in B-cells. When pathogens are present, these follicles expand to reveal prominent germinal centres containing actively dividing, antibody-secreting B-cells;Inner cortex (paracortex) – this is just listed below the external cortex and also is particularly rich in T-cells, which additionally continually circulate throughout most other regions of the node;Medulla – the central inner part of the node the contains large numbers of resolved phagocytic macrophages. These continually monitor the lymph for potentially pathogenic international material (a procedure known together immuno-surveillance), which castle phagocytose top top contact.Clinical significance

During infection, antibody-producing B-cells begin to proliferate in the germinal centres, bring about the impacted lymph nodes to enlarge and also become palpable and tender. Few of the cytokines released are pyrogenic (meaning they cause fever) and act directly on the thermoregulatory centre in the hypothalamus to rise body temperature. As the bulk of person pathogens divide optimally at roughly 37°C, this increase in body temperature offer to sluggish down bacter replication, permitting the epidemic to be dealt with much more efficiently through the immune system. Puffy lymph nodes and a fever are both sure indications that the human body is mounting an effective immune response versus the offending pathogen; this will be disputed in more detail in component 3 that this series.

Other lymphatic components

Other types of lymphatic tissue additionally exist. Mucosa-associated lymphoid organization (MALT) is positioned to safeguard the respiratory and gastrointestinal tracts from invasion by microbes. The complying with are consisted of of MALT:

Gut-associated lymphoid tissue;Bronchus-associated lymphoid tissue;The palatine, lingual and also pharyngeal tonsils.

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The tonsils room aggregates the lymphatic organization strategically situated to prevent foreign material and also pathogens from entering the body. The palatine tonsils space in the pharynx, the lingual tonsils in the dental cavity and also the pharyngeal tonsils (adenoids) are at the ago of the nasal cavity; as a an outcome of this, the tonsils themselves are at high threat of infection and inflammation (tonsillitis). This will likewise be disputed further in component 3.