Lymphoid Organs

The Lymphoid Organs of Immune System

Lymphoid organs are of two types – Primary and Secondary. They are classified based on their function.

The Immune system is like an organization of soldiers or police who protects from foreign or unwanted elements. The immune cells (soldiers) constantly patrol and look for foreign invaders. Immunity is defined as the ability of host to fight pathogenic attack. The immune system consist of proteins, cells and organs.

In this article, we will be discussing different organs that plays an important role in the immune system –

Organs acting like barriers –

The organs namely skin and mucous membranes act like barriers and they are the first line of defense against the pathogens. They restrict their entry.

The skins consist of epidermis (outer layer) and dermis (inner layer). The layers of epidermal cells are found in epidermis. The most outer layer of epidermis consists of dead cells and keratin proteins. The dermis layer consists of sweat and sebaceous glands and hair follicles. They secrete sebum, which inhibits the microbial growth. The breaks, cracks or wounds may allow the pathogenic organism to enter. Hence, the outer layering of skin acts like physical barrier for pathogens. In a similar way, the inner lining of respiratory, urinary or other system produces mucous membrane. The secretions like saliva, tears and mucous also carries antimicrobial enzymes. Cilia, hair like structure are lined inside the respiratory system. Its function is entrapping the pathogen. Coughing and sneezing helps to remove the pathogens. Acid of the stomach also kills the pathogens. First line of defense is not always able to protect the host from pathogens and hence there is need for second line of defense.

Lymphoid Organs –

The immune organs are called as lymphoid organs. Based on the role in immune system, they are classified as Primary Lymphoid and Secondary Lymphoid organs. In primary lymphoid organs are Thymus, Bone Marrow and lymphatic system. They are called as primary because they are the site for origin and development of lymphocytes (T and B cells). The secondary lymphoid organs are spleen, tonsils, MALT and GALT. These are the site for the entrapment of pathogens and attack of immune cells.

Primary Lymphoid Organs –

Thymus

Thymus is the primary organ present above the heart. It is the site for development and maturation of T cells. Its function is to produce an army of T cells that could protect the body from various foreign pathogens. The immune system mediated by T cells is called as Cell mediated immune system. It is bilobed ( 2 lobes) and appears flat. Both the lobes are covered with capsule. Each lobe is further divided into lobules. These lobules are separated from each other with connective tissue called trabeculae. Each lobule has two compartments, the outer compartment is called as cortex and inner compartment is called as medulla. The cortex is heavily packed with immature T cells (Thymocytes) and medulla consists of few thymocytes.

The mid region of cortex and medulla consist of network of stromal cells like epithelial cells, dendritic cells and macrophages. These cells create a microenvironment for the growth and maturation of thymocytes. The epithelial cells are found both in cortex and medullar region. These epithelial cells are also called as nurse cells because they surround the developing thymocytes and constantly interact with them.

Lymphoid organ - Thymus

While developing the T cells, they are able to differentiate the self and non self MHC complexes. Thymus destroys the T cells that fail to identify self MHC by apoptosis.

The research showed that when mice are thymectomized (i.e. thymus is removed surgically), there is loss of cell-mediated immune response due the absence of T cells. When congenitally, humans that lack cell-mediated immune response, it is called as DiGeorge’s syndrome. With age, the thymus function declines.

Bone Marrow –

It is found in the center of the most bones. It is the site for origin, growth, development and maturation of B cells. The B cells are originated from lymphoid progenitors (Hematopoiesis). Similar to thymus, the bone marrow also consist of stromal cells, they interact with B cells by releasing cytokines and nurse them. While developing the B cells, they are able to differentiate the self and non-self antigens. Bone marroe destroys the B cells that fail to identify self-antigen by apoptosis (similar to thymus). From species to species, the site for B cell development may varies for example, in birds, the B cells develops and mature in bursa of Fabricius, lymphoid organ present in gut. In mammalian fetal development, the site for B cells development varies from stages to stages. In early stage, it occues in fetal spleen, in later stage it occurs in ileal Peyer’s patch.

Secondary Lymphoid Organs –

Lymph node, spleen and MALT are the secondary lymphoid organs. Lymph node and spleen are more organized organs than MALT.

Introduction to Lymphatic system –

The lymphatic system is like drainage system of our body. It is subsystem of circulatory system and immune system. The circulatory systems’ function is to provide fresh oxygen to the organs and remove the waste. This exchange occurs in smallest blood vessels called as capillaries. The blood plasma carrying nutrients move out via arterial end of capillaries, whereas the waste is removed via venous end. But during this, not all the waste is removed. Around 15% of the waste remains and it is called as lymph. If this residual waste is not removed, it may cause swelling or inflammation. The lymphatic system does this role. It removes the waste and returns to the circulatory system. The lymph from connective tissue moves to tiny lymphatic capillaries and travels to lymphatic vessels and reach the largest lymphatic vessel is thoracic duct. The lymph is then emptied in the left subclavian vein near the heart. The capillaries of lymphatic system are open-ended and allow the lymph to flow in only one direction. The overlapping line of endothelial cells forming one-way valves achieves this.

In circulatory system, the blood is pumped by Heart whereas in lymphatic system, the muscular movement of the body pumps the lymph flow. The lymph flow plays an important role in carrying the foreign and pathogens from body tissue to lymphoid organs. The lymphatic vessels also carry and transport macrophages and lymphocytes. The immune cells may attack and inactivate the foreign molecule and cause its inactivation.

Lymphatic vessel - Lymphoid Organ
Lymphatic Vessel, Source – 5th Edition, Kuby

Lymph Nodes –

Lymph nodes are part of the lymphatic system. They are bean like structure forming network across the body. They are packed with immune cells like T cell, B cells, dendritic cells, macrophages etc. The lymph (carrying pathogen or antigen) carried by capillaries is first brought to lymph nodes. Immune cells present in the nodes attack the pathogen. The lymph consists of three regions – cortex, paracortex and medulla. The cortex region contains primary follicles. Once they come into contact with the antigen they enlarges to secondary follicles. The cortex is followed by paracortex, and this region is packed with T cells and antigen presenting dendritic cells. The inner most part is medulla and it is rich with antibody producing B cells. Hence, lymph nodes trap the antigens from local tissues.

Lymphoid Organ - Lymph node
Cross Section of Lymph node, Source – 5th Edition, Kuby

Spleen –

Spleen is present in the left abdominal cavity. The spleen receives antigen via splenic arteries. Hence, spleen is involved in trapping antigens from blood. The spleen is surrounded by capsule and it is divided into different compartments with trabeculae. The compartment has further two visible region – Red pulp and White pulp region. The two regions are separated by marginal zone. The Red pulp contains web of sinusoids, RBCs, few lymphocytes and macrophages. Red pulp is the site of destroying or killing the defective and old RBCs. White pulp is surrounded to the branches of splenic artery and froms periarteriolar lymphoid sheath (PALS), it is rich with T cells. The primary follicles are found to attach to PALS. They are dense with B cells and germinal center. The region peripheral of PALS is called as marginal zone and they are populated with macrophages and lymphocytes.

The blood carrying antigens enters the spleen via splenic artery and gets emptied in marginal zone. The antigen presenting cells trap the antigen and present to TH cells. These TH further activates the B cells. The activated TH and B cells travels to primary follicles. Because of antigenic attack, the primary follicles develop to secondary follicles containing germinal center.

Lymphoid Organs - Spleen
Cross section of Spleen, Source – 5th Edition, Kuby

Mucosal Associated Lymphoid Tissue –

The mucous membrane lining of urogenital, respiratory and digestive tract is prone to get attacked by the pathogens. These mucous membranes are associated with lymphoid tissues called as Mucosal Associated lymphoid tissue (MALT). The MALT is found as organized and unorganized form. Its unorganized form is found in the intestinal lining and its organized form is tonsils, Payer’s patch and appendix. The MALT is rich in antibody producing B cells.

References –

5th Edition, Kuby, W.H.Freeman & Co Ltd.

https://www.ncbi.nlm.nih.gov/books/NBK279396/

https://www.ncbi.nlm.nih.gov/books/NBK279395/

Dr. Sangha Bijekar has 7 years of Teaching Experience at University level. She loves to get engage in teaching and learning process. She is into blogging from last two years. She intends to provide student friendly reading material. She is avid Dog Lover and animal rescuer. She is learned Bharatnatyam and Katthak Dancer. She is into biking and She also loves to cook.

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