Lymphoid organs are part of the lymphatic system that performs many immunological functions. The lymphatic system is comprised of organs, vessels, lymph nodes, and lymph. Lymphoid organs are mainly of two types-primary and secondary lymphoid organs. The primary lymphoid organs or the central lymphoid organs such as bone marrow and thymus are the site of the maturation of lymphocytes. In contrast, the secondary lymphoid organs, also known as peripheral lymphoid organs like spleen, lymph node, etc. are associated with the initiation of adaptive immune response. In this article, we will explore lymphoid organs, types, and differences between primary and secondary lymphoid organs.
What are Lymphoid Organs?
Definition of Lymphoid Organs: Lymphoid organs can be defined as the specialized tissues or organs in which a type of white blood cells i.e. lymphocytes are developed, matured, and carried out for various immune functions.
In vertebrates, lymphoid organs are specialized types of organs that are part of the lymphatic system or lymphoid system. Lymphoid organs include bone marrow, thymus, spleen, lymph nodes, and other organs involved in the immune system. Lymphoid organs are not only the place where maturation and proliferation of lymphocytes, the main components of the immune system occur. But they also perform many immune functions to protect the body from pathogens (e.g., bacteria, viruses, and parasites) and eliminate diseased cells (e.g., virus-infected or tumor cells).
Types of Lymphoid Organs
Lymphoid organs can be categorized in two ways:
- Based on their involvement in the development of the immune system and generating immune responses, lymphoid organs can be divided into three types:
- Primary Lymphoid Organs: Bone marrow, Thymus, and Bursa of Fabricius in birds.
- Secondary Lymphoid Organs: Spleen, Lymph nodes, Tonsils, Peyer’s patches of the small intestine, and Mucosal-associated lymphoid tissues (MALT)
- Tertiary Lymphoid Organs
- Based on their location, lymphoid organs are of two types:
- Internal Lymphoid Organs – Thymus, Spleen, Bone marrow, and some of the Lymph nodes.
- External Lymphoid Organs – MALT and Lymph nodes associated with them.
Primary Lymphoid Organs and Their Functions
Primary lymphoid organs functions as the sites of development and maturation of B and T-lymphocytes. They are also known as central lymphoid organs. These types of lymphoid organs include the following:
Bone Marrow
- Red bone marrow is a sponge-like tissue within the long bones found in the pelvis, ribs, and breastbones of adults.
- The B-lymphocytes are produced and matured within bone marrow whereas T-lymphocytes travel to the thymus for maturation and then join the B-lymphocytes in the circulatory system.
Thymus
- The thymus is a bi-lobed, pinkish-grey organ present behind the breastbone and above the heart.
- T-lymphocytes develop as hematopoietic precursors in the bone marrow and then migrate to the thymus for maturation. In the thymus, they are known as thymocytes. Thymus secretes thymosin hormone which plays an important role in the proper functioning of the immune system.
Secondary Lymphoid Organs and Their Functions
Secondary lymphoid organs also known as peripheral lymphoid organs are the sites of proliferation and differentiation of B and T-lymphocytes. undergo proliferation and differentiation. These lymphoid organs include the following:
Lymph Nodes
These are small bean-shaped specialized lymphoid tissues through which the lymph flows its way back to the blood. The selection of B lymphocytes occurs in the germinal center of these lymph nodes. They also filter the lymph and prevent foreign particles or germs from entering the bloodstream.
Spleen
This bean-shaped organ is the largest of all lymphoid organs located in the left upper abdomen under the diaphragm. It removes antibody-coated bacteria and damaged blood cells. It is also involved in the synthesis of antibodies.
Tonsils
They are a pair of soft tissue masses located at the back of the throat near the palate region. Palatine tonsils, adenoids, and lingual tonsils are together known as Waldeyer’s rings. They protect the body by filtering bacteria and viruses from blood cells.
Peyer’s Patches
These are small clusters of lymphoid nodules located throughout the mucus membrane of the small intestine, especially along the ileum. They are part of gut-associated lymphoid tissue (GALT). They play an important role in controlling the growth of pathogenic bacteria in the intestines.
Mucosal-Associated Lymphoid Tissues (MALT)
MALT is an aggregate of many lymphoid tissues found in the mucosal lining of respiratory, digestive, and urinogenital tracts and also in the submucosal membranes of salivary glands, thyroid, breast, nasopharynx, lungs, eyes, and skin. They are the center of lymphocyte production.
Tertiary Lymphoid Organs and Their Functions
Tertiary lymphoid organs contain very few lymphocytes and are generally found at the chronic inflammation sites in case of autoimmune diseases such as rheumatoid arthritis. These organs also play an important role in generating immune response to cancer.
Lymphatic Vessels
Lymphatic vessels are a network of thin-walled ducts that interconnect the lymphoid organs through which lymph circulates. They are also called lymph vessels. They maintain the unidirectional flow of lymph through them by several lymphatic mini-valves.
Lymph
Lymph is a clear, yellow-colored extra interstitial fluid consisting of water (about 95% water) along with plasma proteins, glucose fatty acids, ions, lymphocytes, macrophages, and other trace elements. It helps to transport nutrients, lymphocytes, antibodies, metabolic wastes, and cellular debris. It also promotes intercellular communication.
Internal and External Lymphoid Organs
The lymphoid organs again can be classified into two types – internal and external lymphoid organs based on their location within the body. The bone marrow, thymus, spleen, and some lymph nodes are included in internal lymphoid organs whereas all the components of MALT as well as the lymph nodes that come from MALT are included in external lymphoid organs.
The skin plays an important role in the first line of defense and is thus included in external lymphoid organs. These two types of lymphoid organs function independently in host defense. Internal lymphoid organs are responsible for a systemic immune reaction or systemic immunity whereas external lymphoid organs play an important role in local immune reaction or local immunity.
Differences Between Primary and Secondary Lymphoid Organs
Primary vs secondary lymphoid organs:
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Basis of comparison
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Primary Lymphoid Organs
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Secondary Lymphoid Organs
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Definition
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The organs are the sites of maturation and proliferation of lymphocytes.
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The organs where matured lymphocytes are maintained.
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Name of the organs
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Bone marrow and thymus.
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Lymph nodes, spleen, tonsils, MALT (mucosa-associated lymphoid tissue).
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Location
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The organs are located in certain places like bone marrow within certain bones.
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The organs are distributed throughout the body like lymph nodes are found within the lymph vessels throughout the body.
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Role
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These organs allow immune cells to originate, proliferate, and mature.
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These organs help to generate an immune response.
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Types of Lymphocytes
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Either T cells or B cells are present.
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Both T cells and B cells are present.
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Interaction with antigens
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No direct interaction with antigens.
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Direct interaction with antigens.
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Effect of ageing
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Organs undergo atrophy with age.
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Organs increase in size with age.
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What is the Function of the Lymphoid Organs?
The functions of lymphoid organs are stated below:
- Lymphoid organs play an important role in the development and normal functioning of the immune system.
- The primary lymphoid organs such as the bone marrow and thymus are the sites of proliferation and maturation of B-lymphocytes and T-lymphocytes.
- Bone marrow is also involved in the production of blood cells.
- The spleen is a hematopoietic organ in the fetus. In adults, it helps to remove infectious pathogens and damages blood cells.
- Lymph nodes and spleen also help in filtering out the unwanted lymphocytes.
- The tonsils prevent the entry of foreign particles and pathogens into the body.
Diseases Associated with Lymphoid Organs
Many diseases and disorders often affect the lymphoid organs. Some of these diseases are stated below:
- Nezelof Syndrome: This congenital disorder results in atrophy of the thymus.
- Lymphoma: Hodgkin lymphoma and non-Hodgkin are the cancer of lymph nodes.
- Lymphadenopathy: It causes the swelling of lymph nodes.
- Lymphadenitis: It occurs when lymph nodes are infected by bacteria, fungi, or viruses
- Lymphangitis: It is the inflammation or swelling of the lymphoid organs due to bacterial, fungal, or viral infections.
- Tonsilitis: Bacterial and viral infections result in swelling of tonsils which is termed tonsilitis.
Conclusion – Lymphoid Organs
Lymphoid organs are the sites of differentiation, proliferation, and maturation of immune cells. The two main types of lymphoid organs are primary (such as bone marrow and thymus) and secondary (such as the spleen, tonsils, etc.) work together to provide immunity to the body. These lymphoid organs along with lymphatic vessels and lymph form the lymphatic system or lymphoid system.
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FAQs on Lymphoid Organs
What are the 7 Organs of the Immune System?
The primary lymphoid organs of the immune system are the thymus and bone marrow. The secondary lymphoid organs include the spleen, tonsils, lymph vessels, lymph nodes, adenoids, skin, and liver.
Which is not a Lymphoid Organ?
The organs that are not a part of the lymphatic system are not lymphoid organs. The appendix is not a lymphoid organ.
What is the Largest Lymphoid Organ?
The spleen is the largest lymphoid organ in the human body. It is located in the left upper quadrant of the abdomen and just beneath the diaphragm.
What are the Lymphoid Organs?
Lymphoid organs are the specialized organs of the lymphatic system. These organs include bone marrow, thymus, spleen, tonsil, lymph nodes, Payer;s patches, and MALT.
Why are they Called Lymphoid Organs?
Lymphoid organs are composed of lymphoid tissue and are the sites either of lymphocyte production or activation. The cells of the lymph are mostly lymphocytes. Therefore, the organs are called lymphoid organs.
Which of the Following are Primary Lymphoid Organs?
The primary lymphoid organs include the bone marrow and the thymus gland.
Name the Primary and Secondary Lymphoid Organs?
The primary lymphoid organs are the bone marrow and the thymus gland. Secondary lymphoid organs include the lymph nodes, spleen, and tonsils.
What is the Difference Between Lymphoid Tissue and Lymphoid Organs?
Lymphoid tissue is composed of aggregates of lymphocytes and other immune cells dispersed throughout the body, whereas lymphoid organs are specialized structures where lymphocytes mature, proliferate, and carry out immune responses.
What are the 6 Lymphatic Organs?
The six lymphatic organs include the bone marrow, thymus, spleen, tonsils, lymph nodes, and Peyer’s patches in the small intestine.
What are the Peripheral and Secondary Lymphoid Organs?
Peripheral lymphoid organs include lymph nodes, tonsils, and Peyer’s patches, while secondary lymphoid organs encompass the spleen and lymph nodes.
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