What Are The Main Organs Of The Immune System?

The immune system is a fascinating, complex network of organs, lymph vessels, and immune cells working in tandem as a defense system to protect us from foreign invaders and disease. Immune system organs produce, and are responsible for, the maturation of vital defense cells. What are the main organs of the immune system and their lifesaving functions?

What Are The Main Organs Of The Immune System - Graphic Of Internal Organs

Primary Lymphoid Organs Of The Immune System

Immune cells are formed and matured from stem cells in the primary lymphoid organs, namely the bone marrow and thymus gland.


The thymus, a two-lobed, triangular-shaped endocrine gland, is located in the upper thorax behind the sternum. In children, the thymus gland is fully developed, and is quite large. It produces two hormones, thrymosin and thymopoietin, which promote and regulate the maturation of T lymphocytes in the lymph nodes. Following adolescence, the thymus gradually turns into fatty tissue.

Certain immune cells, including T lymphocytes, are differentiated in the thymus. These immune cells have the task of coordinating the adaptive and innate immune systems. T cells are matured in the thymus, in fact, the “T” in T cells stands for “thymus.” The function of the thymus is to train and develop these cells to defend the body from outside invaders, including viruses, bacteria, and fungi.

Bone Marrow

Bone marrow is a gelatinous, spongy tissue found in the center of most major bones. It can be found in flat bones, including the pelvis, skull, and sternum, and long bones, such as the femur and humerus. There are two types of bone marrow, red bone marrow or myeloid tissue, and yellow bone marrow or fatty tissue. Both types are rich in blood vessels and capillaries, which supply oxygen and aid in the transportation of stem cells, which are newly formed blood cells.

Stem cells are produced in the bone marrow, and when fully developed, are released into the bloodstream. All cells within the body are made in the bone marrow, which produce millions of new cells every day, including both red and white blood cells and platelets, or blood-clotting cells. Mesenchymal stem cells, made in yellow bone marrow, produce bone, cartilage, and fat. Red bone marrow contains stem cells, called hematopoietic cells, which give rise to other blood cells via a process called haematopoiesis.

Leukemia and lymphoma, diseases involving bone marrow, are so deadly because they destroy stem cells, and disrupt the bone marrow from making new cells. During a bone marrow transplant, stem cells are collected from the pelvic bone of a donor, and transplanted into a person with cancer to replace diseased bone marrow, in the hopes that the body can produce enough blood cells to prevent infection and regain health.

[Read More: What Is The Immune System?]

Secondary Lymphoid Organs Of The Immune System

Immune cells in the secondary lymphoid organs perform the difficult task of fending off pathogens. Secondary organs include the spleen, lymph nodes, tonsils, and both mucus and gut-associated lymphoid tissue.


Ask anyone what the heart, stomach, and lungs do, and they can tell you several facts about what each organ does. Then ask them if they know what the spleen does and where it’s located. Most people have a general idea where it is in the body, but don’t know what its functions are. The spleen is located beneath the diaphragm, in the upper left side of the abdomen. It’s a multi-tasking organ that continually has a high volume of blood flowing through it.

Before birth, the spleen primarily produces immune cells and manufactures blood. After birth, its main function is to remove blood cells for specific purposes related to immunity. The spleen is also a storage site for immune cells before they’re released into the bloodstream, including T and B lymphocytes.

Platelets, or thrombocytes, are stored and removed in the spleen. Since it’s made of very soft tissue, the spleen can rupture quite easily if injured. Fortunately, other organs can compensate for its tasks in the event it has to be removed.

Organs Of The Immune System - Internal Organs

Lymph Nodes

The lymphatic system contains vessels and lymph nodes, which have the primary task of exchanging fluid, waste, and nutrients between the bloodstream and tissues.

Lymph vessels drain excess fluid from the blood vessels and lymph nodes, kidney-shaped organs that function as biological filter stations to filter lymph before it reaches the larger lymphatic vessels, and ultimately the superior vena cava, where it empties into the bloodstream.

Lymph nodes are considered secondary lymphoid organs, and are interspersed throughout the entire body, clustering in major locations like the armpits, neck, groin, abdomen, and chest.

Nodes – up to 600 of them – are connected by lymph vessels. Lymph fluid contains B and T lymphocytes, immune cells that are part of the adaptive immune system. Both B and T cells originate in the bone marrow, but T lymphocytes mature in the thymus, whereas B lymphocytes mature in the bone marrow.

These immune cells are a first-line defense against antigens and invaders. T lymphocytes are involved in cell-mediated immunity, and destroy pathogens directly, while B lymphocytes mount a humoral, or antibody-mediated defense. Swollen lymph nodes are a sign that the body is mounting a defense reaction against infection, and can signal life-threatening conditions, such as cancer.


Because of their location, immune cells in the tonsils, which are lymph nodes, immediately activate the immune system by preventing germs in the mouth from gaining access to the body. Tonsils are a primary defense against infections of the gastrointestinal and respiratory systems. Tissue within the tonsils contain lymphocytes, which fight infection.

Tonsils are comprised of two palatine tonsils, one on either side, of the throat. Lymphocytic B and T cells are stored within a network of crypts, storage sites for immune cells. The tonsils also produce antibodies against disease. Additionally, there is the lingual tonsil located at the base of the tongue, the adenoids located above the roof of the throat, and lymphatic tissue on either side of the throat that protect against pathogens.


Lymphatic tissue in the mucus membranes, referred to as mucosa-associated lymphoid tissue (MALT), also provides defense against antigens. It is the most extensive component of lymphoid tissue. For instance, if viruses or bacteria try to gain entrance into the body, tissues in the urinary tract or airways mount an attack to prevent them from taking hold. MALT tissue is found in the GI tract, thyroid, salivary glands, eyes, nose, bronchi, and skin.


Gut-associated lymphoid tissue is a component of MALT. Its primary function is to protect the body from invasion by providing a mucosal barrier in the intestinal tract. GALT lies throughout the intestines, comprising about 80% of the immune system and 70% of lymphocytes.

Gut-associated lymphoid tissue is found in the lamina propia, or mucosa of the gastrointestinal tract, in the appendix, the tonsils, the mesenteric lymph nodes, and Peyer’s patches, which are organized lymphoid nodules in the intestinal epithelium.

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Key Points

The immune system is truly an incredible, complex network of organs, immune cells, and lymphoid tissue that work together to defend against viruses, bacteria, and other invaders. The primary lymphoid organs, including the thymus and bone marrow, manufacture and mature stem cells before releasing them into the bloodstream.

The spleen, tonsils, lymph nodes and lymphoid tissue in the mucus membranes and intestines are secondary lymphoid organs that defend the body against pathogens. Did you know the body had so many immune organs? Please share your thoughts in the comments:)



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(1) NCBI: What are the organs of the immune system?

(2) healthline: Palantine tonsil

(3) eMedicine: Mucosa-Associated Lymphoid Tissue Lymphomas (MALTomas)

(4) Medical News Today: All you need to know about bone marrow

(5) WiseGEEK: What Is Bone Marrow

(6) healthline: What Is a Bone Marrow Transplant?

(7) WebMD: What Are Lymph Nodes?

(8) Science Direct: Gut-Associated Lymphoid Tissue

(9) National Library of Medicine National Center for Biotechnology Information: The Thymus: A Forgotten, But Very Important Organ

(10) National Library of Medicine National Center for Biotechnology Information: Biology of human bone marrow stem cells

(11) MedlinePlus: Spleen Diseases

(12) NCBI: Anatomy, Lymph Nodes

(13) NCBI: How do the tonsils work?

(14) National Library of Medicine National Center for Biotechnology Information: The enigma of the lower gut-associated lymphoid tissue (GALT)

(15) US National Library of Medicine National Institutes of Health: Mucosa‐associated lymphoid tissue (MALT) lymphoma: a practical guide for pathologists

(16) NCBI: What are the organs of the immune system?


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14 thoughts on “What Are The Main Organs Of The Immune System?”

  1. Very well detailed information that you have shared here and I must say that I truly find this very worthy. In all honesty, this is really great to see because the immune system has always been a major myth to me though a point of interest but I never truly gave it enough time to learn about. Thanks for sharing so much knowledge about it and the operations plus its constituent parts. Thanks

  2. I knew that lymph vessels drained excess fluid from the blood vessels, and that the lymph nodes, kidney-shaped organs, function as biological filter stations to filter lymph before it reaches the larger lymphatic vessels, and ultimately the superior vena cava, where it empties into the bloodstream. 

    The immune system is truly an incredible, complex network of organs, immune cells, and lymphoid tissue that work together to defend against viruses, bacteria, and other invaders. Thanks for sharing.

  3. You know, I have been told about how the inmune system works and how important it is since I started a new medication for my psoriasis a couple of months ago, and if I’m being honest, I had no idea what the immune system was as a whole. The one thing that surprised me most was that the bone marrow is actually part of the immune system. For some reason, I thought the inmune system was all placed everywhere throughout the organs of your stomach to the simpler ones, but I never imagined all of this. Thank you for the info! I still have a lot to learn but I’m glad I got some straight facts. 

    • Hi Stephanie,

      I’m sorry to hear about your psoriasis. I’m fairly confident that most people don’t understand the complexities of the immune system. Yes, bone marrow is a primary lymphoid organ that products blood cells. It’s interesting how the organs work together. Thanks for taking the time to leave a comment:)

  4. Many thanks to you for sharing such an excellent article with us .The immune system acts as a defense system to protect us disease. It’s made up of organs, lymph vessels, and immune cells that work together.  The thymus trains and develops cells, protecting the body from external attacks including viruses, bacteria and fungi. 

    All the cells inside the body are made up of bone marrow that produce millions of new cells every day in my body. The immune system is truly an incredible, complex organ that work together to protect us. 

    Lastly, I hope that all of you will get to know the organs of the immune system through this article, which will certainly share with you new experiences as you gain important knowledge.

    • Hi Shanta,

      Thanks for reading my article on the immune system. It’s pretty amazing, isn’t it?! I appreciate your comment:)

  5. I have always been interested by the immune system. Since I was young, I was taught that the more you play outside, the dirtier you get, and the better your immune system would become. I am glad I found this article, I was never sure which organs did what to prevent sickness. The Spleen, Lymph Nodes, Tonsils, MALT, and GALT did so many more defensive tasks then I ever would have imagined.

    The spleen takes out blood from the blood stream, I can now see why people can suffer from spleen injuries. If someone’s blood isn’t filtered right, there are bounds of problems to follow. Thank you for this post, I hope others can learn as much as I have today.

    • Hi Kenny,

      Thanks for you comment. The immune system is so interesting its in design and function. It’s such a complex defense system. I don’t many people realize how many organs there are within the immune system. 

       Dirt is teeming with bacteria that can be good for health, which is probably why you’ve heard that it’s good for the immune system. Yes, the spleen can be injured, and if taken out, the other organs will compensate. Thanks so much for reading:)

  6. Hello,

    That’s quite an eye-opening article that you have right there.

    I at least knew that the bone marrow was a primary contributor to the immune system, but I certainly didn’t know that there were so many other organs and systems that do the work collectively.

    I have always thought that tonsils were just troublesome glands (right now they are painful) but now I know they are a defense mechanism. I’ve been also having a painful tongue near the throat- little did I know it’s related to tonsils until I read this article. I’ll do my due diligence and seek treatment if I don’t heal quickly, thanks so much.

    • Thank you. Bone marrow, along with the thymus gland, are primary lymphoid organs as they produce and mature blood cells that are then released into the bloodstream. It’s amazing how all the organs in the immune system work together, and can compensate for each other if one organ is taken out, namely the spleen and tonsils. 

      Yes, the tonsils because of their location are a first-line defense against pathogens that enter the mouth. I’m sorry to hear yours are hurting. I appreciate your reading my article:)

  7. Hi!

    Thanks for the nice review of the immune system’s organs.

    Good organs are needed for good health. We should care about our organs. A person’s body works in the care of organs. You have mentioned some organs and their work. Organs, cells, tissues all work together to maintain our bodies.

    At present, I am reading in the zoological department. I thought I had enough knowledge of biology. But reading your article I found some extra knowledge. That is why I have signed up to your website for getting new posts of new knowledge.

    Thank you again for the nice post. The review is informative as well as educative. One can learn new things through your post.

    • Thank you. Yes, the organs of the immune system work together to keep us healthy. How the immune system works is truly fascinating. 

      I’m glad you learned something from my article. Thank you for signing up to my newsletter and for taking the time to leave a comment:)


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