The lymphatic system in vertebrates consists of lymphoid organs, lymphatic vessels and lymphoid tissues. They form an important part of the immune system and are complementary to the circulatory system. In this article, we are going to study about the lymphoid organs and their types.
Classification of Lymphoid Organs
There are two classifications of lymphoid organs –
- On the basis of roles played by the organs in the development of the immune system or to trigger a response, there are two kinds of organs:
- Primary Lymphoid Organs
- Secondary Lymphoid Organs
- On the basis of location, lymphoid organs are of two kinds:
- Internal Lymphoid Organs
- External Lymphoid Organs
Primary Lymphoid Organs
The organs in which the production of the cells of the immune system occurs are known as primary lymphoid organs. They are also knwon as central lymphoid organs. They include the thymus and the bone marrow.
Thymus
The thymus is a gland-like organ that is situated at the back of the heart and above the breastbone. It is the site for the maturation of thymus cell lymphocytes, also known as T cells. The thymus gives a suitable environment for the production of T cells from hematopoietic progenitor cells.
This gland reaches its full maturity during childhood. As adulthood commences, it is replaced by fatty or adipose tissue. The T cells coordinate the innate and adaptive immune systems.
Bone Marrow
Bone marrow, a primary lymphoid organ, is a spongy tissue found in the bones. It is the site for the production of precursors of T cells, and production and maturation of B cells. These cells are then transported to other organs and tissues via blood to carry out their function. The red bone marrow is found in abundance during the time of birth and it actively produces immune system cells, but over time, the red bone marrow converts to fatty tissues. In adults only the pelvis, ribs and breastbone have red bone marrow.
The B cells that are produced and matured in the bone marrow immediately travel to the circulatory system in search of pathogens. T cells travel to the thymus for maturation and they join B cells in the circulatory system in search of pathogens. T cells are also involved in the process of apoptosis.
Secondary Lymphoid Organs
The secondary lymphoid organs are also known as peripheral lymphoid organs. The function of secondary lymphoid organs is the activation of immune cells and to start a fight against foreign substances in our body. It includes spleen, lymph nodes, tonsils and various mucosal membranes.
Spleen
The spleen is an organ that is found in all vertebrates. It is found in the left upper abdomen, below the diaphragm. The spleen performs various kinds of functions such as:
- It houses different kinds of immune system cells such as lymphocytes and phagocytes. They are transported to other organs via blood when required.
- It breaks down the RBCs. In newborns, the spleen synthesises RBCs, but soon bone marrow takes over the process of hematopoiesis.
- It stores and breaks down platelets, which are important for blood clotting.
- The phagocytes act as a filter for germs or microbes that enter the bloodstream.
Lymph Nodes
The lymph nodes are bean-shaped tissues found along the lines of lymphatic vessels. They act as filters that trap the germs that lead to activation of antibodies in the blood. There are around 800 lymph nodes found in our body, out of which 300 are found in the head and neck region. Lymph nodes are also found in the underarm and abdominal areas.
Tonsils
The tonsils that are located at the throat region form an important part of the immune system. The location of the tonsil is such that it prevents germs from entering the mouth and nose. They have white blood cells that help in fighting the germs.
Mucosal Membranes
The mucosal associated lymphoid tissue (MALT) is a small concentration of lymphoid tissue found in submucosal membranes such as thyroid, breast, nasopharynx, salivary glands, gastrointestinal tract, eyes and skin. They contain B cells, T cells and macrophages that encounter germs or pathogens that enter the mucosal epithelium.
Tertiary Lymphoid Organs
Tertiary Lymphoid Organs (TLOs) are found at the site of chronic inflammations, graft rejections, or some cancers. They are not as densely populated with lymphocytes as primary and secondary lymphoid organs, but they elicit immune responses when inflammatory conditions arise.
Internal and External Lymphoid Organs
The internal lymphoid organs include the thymus, spleen, bone marrow and some of the lymph nodes. On the other hand, the external lymphoid organs include the MALT and lymph nodes associated with them. The skin is also an external lymphoid organ but it does contain organised lymphoid tissues. Skin has immune components that are directly associated with immune response.
The immune response elicited by internal lymphoid organs is called systemic immune reaction or systemic immunity, whereas that by external lymphoid organs is referred to as local immune reaction or local immunity.
This sums up briefly on lymphoid organs. Visit BYJU’S Biology for more such related information.
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