Primary and secondary lymphoid organs detailed study: We can categorize all lymphoid organs according to their functions in the immune system- primary lymphoid organs and secondary lymphoid organs.
Primary lymphoid organs:
Two types of lymphoid organs are primary in nature – bone marrow and thymus.
Bone marrow: In bone marrow different types of body cells are produced. Example: Red blood cells. All lymphocytes are produced in bone marrow. So bone marrow is the production house for all lymphocytes. After production some lymphocytes differentiated in bone marrow. These lymphocytes which differentiated in bone marrow are popularly known as B lymphocytes. Other lymphocytes get differentiated in thymus. So in the immune system bone marrow is responsible for production of all lymphocytes and differentiation of B lymphocytes.
Thymus: Thymus is a lobed primary lymphoid organ. It presents the location nearby. Thymus is the only lymphoid organ where differentiation of T lymphocyte occurs. T lymphocytes are produced in bone marrow and after that they reach to thymus to become differentiated. So for development both bone marrow and thymus are important in case of T lymphocyte.
Without thymus it is impossible for T lymphocytes to develop completely. Immature T cell, thymocytes go through different stages in the thymus and become fully developed T lymphocytes. Thymocytes generate T cell receptors. T cell receptor or TCR is a unique antigen receptor.
Based on these TCR thymocytes are selected for further process. Selection occurs in two ways, position selection and negative selection. At negative selection thymocytes die. More than ninety five percent thymocytes die. Only a few thymocytes become successful in positive selection. At the time of birth its size is larger than its size during puberty. It decreases its size with time.
Secondary lymphoid organs:
Secondary lymphoid organs are more in number than primary lymphoid organs. Major secondary lymphoid organs are – lymph nodes, spleen and mucosal associated lymphoid tissue.
Lymph Nodes: In the lymphatic system there are around 160 solid fibrous structures. These solid structures are made of connective tissue fibres. They are encapsulated in highly specialised secondary lymphoid tissue. They are made of stromal cells. Lymph nodes have three layers. Outer cortex, middle paracortex and inner medulla layer. B lymphocytes, dendritic cells and macrophages present in the cortex region.
In the paracortex region T lymphocyte and dendritic cells are present. In the medulla region plasma cells are present. It acts as a filter in the path of lymph flow. They are like a filter for external microbes. Different types of engulfing cells present at lymph nodes take action on external microbes. Lymph nodes filter the tissue fluid and trap the microbes and antigen of harmful nature. Then the adaptive immunity system becomes active and removes external microbes.
Spleen: It is a large oval shaped major secondary lymphoid organ which performs the same function as lymph nodes do. The spleen has other functions also. But in immunity function they trap the microbes and trigger adaptive immunity as that of lymph nodes.
Lymphocytes and phagocytes present in the spleen are responsible for immune response. It is specialised for the antigen which is blood borne. So the spleen is responsible for immune response for systemic infections. Spleen have three different microenvironments, red pulp, white pulp and follicles.
Mucosal Associated Lymphoid Tissue: More than fifty percent lymphoid tissue present in mucosal associated lymphoid tissue or MALT. They are the tissues which present in all organs lining with mucosa. In different body parts, like gastrointestinal tract, respiratory system tract, urogenital tract mucosal associated lymphoid tissue present.
Mucosal associated lymphoid tissue gives immunity to those portions of the body. Mucosal associated lymphoid tissue gives protection against outer enemy microbes to those tissues.
Peyer’s patches and appendix also act as secondary lymphoid organs.
Read More: Function of lymphatic system detailed study
Reference: Primary and secondary lymphoid organs detailed study
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