Lymphoid Organs: Learn Definition, Types and Functions

Lymphoid Organs: Do you know what our immune system is made up of? Can you name one organ or a body part that protects our body from pathogens? Our immune system comprises many individual cells, tissues, proteins (antibodies), and many organs called lymphoid organs.

The Lymphoid organs are the sites where the maturation and proliferation of lymphocytes occur and help carry out various immune functions. Lymphoid organs form a part of the lymphatic system composed of lymphatic vessels, lymph nodes, and lymph. Know everything about primary and secondary lymphoid organs from this article below.

Lymphoid Organs Definition

Lymphoid organs are defined as those organs where the maturation and proliferation of lymphocytes take place. They also play a very important role in the development of immune cells and also act as the checkpoints of foreign substances.

Or in other words, the Lymphoid organs are specialized tissues that provide the typical anatomic location and microenvironment, which helps in attaining maturity of lymphocytes and thus, activates them.

Lymphocytes are a type of WBC (leucocytes) that are the main components of our immune system. There are two types of lymphocytes, T-lymphocytes, and B-lymphocytes.

Fig: Major Lymphoid Organs in Human

Types of Lymphoid Organs

The lymphatic system is composed of three types of lymphoid organs which are as follows:
1. Primary lymphoid Organs
2. Secondary lymphoid Organs
3. Tertiary lymphoid Organs

1. Primary Lymphoid Organs
I. Primary lymphoid organs are those organs where B and T-lymphocytes mature and acquire antigen-specific receptors.
II. These organs are the sites of origin and proliferation of lymphocytes. Thus, they are also called central lymphoid organs.
III. After the maturation of lymphocytes, they migrate to the secondary lymphoid organs.
IV. These organs include bone marrow and thymus (and the bursa of Fabricius in birds).
a) Bone Marrow
1) It is a sponge-like tissue found inside the long bones.
2) The red bone marrow is involved in the immune system working.
3) It is the main lymphoid organ where all blood cells, including lymphocytes, are produced and multiply.
4) At the time of birth, many bones contain red bone marrow, which actively creates immune system cells. In adulthood, only a few of our bones contain red bone marrow, including the ribs, breastbone, and pelvis.
5) Bone marrow is primarily responsible for both the creation of T-cell precursors and the production and maturation of B cells.
6) Maturation of B-lymphocytes completes in the bone marrow only. On the contrary, T-cells travel from the bone marrow to the thymus, where they develop further and mature.
7) From the bone marrow, B cells immediately join the circulatory system and travel to secondary lymphoid organs in search of pathogens.

Fig: Bone Marrow

b) Thymus
1) The thymus is a bi-lobed, pinkish-grey organ located behind the breastbone and above the heart.
2) It is quite large in size at the time of birth but keeps reducing with age. It reaches full maturity only in children and then slowly transforms into fatty tissue.
3) It is the site where T-lymphocytes mature. T-cells begin as hematopoietic precursors from the bone marrow and eventually migrate to the thymus, where they are referred to as thymocytes.
4) T-cells mediate the cell-mediated immune response (CMIR).
5) Thymus also secretes a family of hormones collectively known as thymosin which is essential for the normal functioning of the immune system.

Fig: Structure of Thymus

2. Secondary Lymphoid Organs
I. Secondary lymphoid organs are the sites where B and T-lymphocytes undergo proliferation and differentiation.
II. They interact with the antigens and become effector cells. They are also called the peripheral lymphoid organs.
III. They are associated with the initiation of the adaptive immune response.
IV. In the secondary lymphoid tissues, the lymphocytes move from one lymphoid organ to another through blood and lymph.
V. These organs include lymph nodes, spleen, tonsils, Peyer’s patches of the small intestine, and Mucosal associated lymphoid tissues (MALT).
a) Lymph Nodes
1) These are small bean-shaped tissues found at intervals along the lymphatic system composed of lymphoid tissue.
2) These are an organized collection of lymphoid tissue, through which the lymph passes on its way back to the blood.
3) These are the first organized lymphoid structures that encounter the antigens entering the tissue spaces.
4) In the germinal centre of these lymph nodes, the selection of B lymphocytes occurs.
5) They act as filters for the lymph, preventing foreign particles or germs from entering the bloodstream and also activate the production of special antibodies in the blood. It also produces lymphocytes and plasma cells.
6) Swollen or painful lymph nodes indicate that the immune system is active, i.e., fighting against an infection.

Fig: Structure of Lymph Node

b) Spleen
1) It is a large bean-shaped organ located in the left upper abdomen and beneath the diaphragm. It is the largest lymphoid organ. This appears due to more blood flow through it.
2) In the foetus, the spleen produces all types of blood cells, but in adults, it only produces lymphocytes and phagocytes.
3) It is a large reservoir of erythrocytes (or RBCs). At maturity, it removes particulate matter and aged blood cells, mainly red blood cells.
4) The spleen is involved in the synthesis of antibodies in its white pulp. By way of blood and lymph node circulation, the spleen is also involved in the removal of antibody-coated bacteria and antibody-coated blood cells.
5) If the spleen is removed completely, then the other immune systems can carry out their functions.

Fig: Anatomy of Spleen

c) Tonsils
1) The tonsils (also called palatine tonsils) are a pair of soft tissue masses located at the rear of the throat (pharynx).
2) Since they are located at the throat and the palate regions, they can stop the germs from entering the body through the mouth or the nose.

3) There are different types of tonsils like palatine tonsils, adenoids, and the lingual tonsil, which are sometimes together called Waldeyer’s ring because they form a ring around the opening to the throat from the mouth and nose.
4) They act as filters to protect the body from bacteria and viruses and also contain white blood cells, which are responsible for killing germs.

Fig: Types of Tonsils

d) Peyer’s Patches
1) Peyer’s patches are small clusters of lymphoid follicles found throughout the mucus membrane of the small intestine, especially along the ileum. They are also called aggregated lymphoid nodules.
2) These are an important part of gut associated lymphoid tissue (GALT). They are located within both the small and large intestines of many mammalian species.
3) Pathogenic microorganisms, any form of antigens, etc. which enter the intestinal tract come across the macrophages, dendritic cells, B-lymphocytes, and T-lymphocytes found in Peyer’s patches.
4) T cells, B-cells and memory cells are stimulated when antigens come in contact with Peyer’s patches.
5) They play an important role in the immune system by monitoring the intestinal bacteria populations and thus preventing the growth of pathogenic bacteria in the intestines.

Fig: Peyer’s Patches

e) Mucosal-Associated Lymphoid Tissues (MALT)
1) MALT is a significant aggregate of lymphoid tissues located in the mucosal lining of the major tracts like respiratory, digestive, and urinogenital tracts.
2) MALTs are concentrations of many lymphoid tissues found in the various submucosal membrane sites of the body, like the gastrointestinal tract, nasopharynx, thyroid, breast, lung, salivary glands, eye, and skin.
3) It constitutes about \({\rm{50 \% }}\) of the lymphoid tissues in the human body.
4) They do not serve as the filters of lymph but are the centres of lymphocyte production.
5) Apart from B-lymphocytes and T-lymphocytes, phagocytic macrophages and dendritic cells are also present.

Fig: Mucosal Associated Lymphoid Tissue (MALT)

3. Tertiary Lymphoid Organs
I. Tertiary lymphoid organs are generally found at the sites of chronic inflammation in autoimmune diseases (like rheumatoid arthritis).
II. These organized accumulations of T and B cells basically resemble secondary lymphoid organs and generate autoreactive effector cells.
III. Tertiary lymphoid organs usually contain very few lymphocytes that assume their role when they encounter the antigens that cause inflammation.
IV. These organs also play a prominent role in the immune response to cancer.

Fig: Tertiary Lymphoid Structure in Cancer

What are the Lymphoid Organs Functions?

The functions of lymphoid organs are as follows:
1. The main function of lymphoid organs is developing and providing immunity to the body.
2. The primary lymphoid organs, i.e., the bone marrow and thymus, are the sites where the proliferation and maturation of B-lymphocytes and T-lymphocytes take place.
3. Bone marrow is involved in the production of blood cells, i.e., RBCs (or erythrocytes), WBCs (or leucocytes), and Platelets (or thrombocytes).
4. The spleen helps in the removal of damaged red blood cells. In foetal conditions, this is also a haematopoietic organ.
5. Lymph nodes and spleen helps in filtering out and destroying the unwanted lymphocytes. They also help in maintaining the population of mature lymphocytes to enable the adaptive immune response to begin.
6. The tonsils prevent foreign materials and pathogens from entering the body.

Summary

Lymphoid organs are the sites where the maturation and proliferation of lymphocytes take place. Their main function is to develop and provide immunity to the body. There are two main types of lymphoid organs, primary (like bone marrow and thymus) and secondary (like spleen, tonsils, etc.). While primary lymphoid organs are the sites of origin and proliferation of lymphocytes, the secondary lymphoid organs are the sites where B and T-lymphocytes undergo proliferation and differentiation.

Frequently Asked Questions (FAQs) on Lymphoid Organs

Let’s look at some of the commonly asked questions about lymphoid organs:

Q.1: What are the functions of Lymphoid Organs?
Ans: 1. It helps in developing and providing immunity to the body.
2. Bone marrow is involved in the production of blood cells, i.e., RBCs (or erythrocytes), WBCs (or leucocytes), and Platelets (or thrombocytes).
3. Bone marrow and thymus are the sites where the proliferation and maturation of B-lymphocytes and T-lymphocytes take place.

Q.2: What are the types of lymphoid organs?
Ans: 1. Primary lymphoid organs
2. Secondary lymphoid organs
3. Tertiary lymphoid organs

Q.3: Which is the body’s largest lymphoid organ?
Ans: Spleen is the largest lymphoid organ.

Q.4: What are the major lymphoid organs?
Ans: The major lymphoid organs are bone marrow, thymus, spleen, tonsils, lymph nodes, Peyer’s patches, and mucosal-associated lymphoid tissue (MALT).

Q.5: What are lymphoid organs made up of?
Ans: Lymphoid organs are made up of lymphoid tissue, which are the sites of lymphocyte production and maturation.

We hope this detailed article on Lymphoid Organs helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest. We have already mentioned about the difference between primary and secondary lymphoid organs in this blog.