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Lymphatic System and its Components: The human immune system consists of lymphoid organs, tissues cells and soluble molecules like antibodies !
Immune system recognises foreign antigens responds to these and remembers them.
This system also plays an important role in allergic reactions, autoimmune diseases and organ transplantation.
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Lymphoid Organs:
Lymphoid organs are those organs where the maturation and proliferation of lymphocytes takes place.
Types of Lymphoid organs:
There are two types of lymphoid organs: primary lymphoid organs and secondary lymphoid organs.
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1. Primary lymphoid organs:
The primary lymphoid organs are those organs where T lymphocytes and В lymphocytes, mature and acquire their antigen-specific receptors. After maturation, the lymphocytes migrate to secondary lymphoid organs. Primary lymphoid organs include bone marrow and thymus.
(i) Bone marrow:
Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are formed. Maturation of B-lymphocytes occurs here.
(ii) Thymus:
Thymus is the site of T lymphocyte maturation. Thymus is situated near the heart. Thymus is quite large in size at the time of birth but keeps reducing with age. As stated earlier, T-lymphocytes and B-lymphocytes are responsible for cellular and humoral immune response respectively.
2. Secondary lymphoid organs:
After maturation, В lymphocytes and T lymphocytes migrate via blood vascular and lymphatic system to the secondary lymphoid organs where they undergo proliferation and differentiation. The acquired immune response to antigens usually develops in these organs and become effector cells.
In the secondary lymphoid tissues, the lymphocytes do not remain, and move from one lymphoid organ to another through blood and lymph. The secondary lymphoid organs are lymph nodes, spleen, tonsils, Peyer’s patches of the small intestine and mucosal associated lymphoid tissues (MALT).
(i) Lymph nodes:
These are small solid structures found at intervals along the lymphatic system. They are composed of lymphoid tissue and act as filters for the lymph, preventing foreign particles from entering the bloodstream. Lymph nodes also produce lymphocytes and plasma cells.
(ii) Spleen:
It is a bean shaped organ which is the largest single mass of lymphoid tissue in the body. In foetus the spleen produces all types of blood cells but in adult it only produces lymphocytes. Macrophages of spleen are phagocytic.
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(iii) Tonsils:
Usually there are six tonsils. They act as filters to protect body from bacteria and aid in the formation of white blood cells.
(iv) Peyer’s patches:
These are clusters of lymph nodules found in small intestine, especially along the ileum. They produce lymphocytes.
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(v) Mucosal-Associated Lymphoid Tissues (MALT):
MALT are significant aggregations of lymphoid tissues which are seen in relation to the mucosa of the major tracts like respiratory, alimentary canal and urinogenital tracts. It constitutes about 50 percent of the lymphoid tissue in human body.
They do not serve as filters of lymph. Larger aggregations extend into the submucosa. However, they are centres of lymphocyte production. Apart from B-lymphocytes and T- lymphocytes, phagocytic macrophages and dendritic cells are present.
