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Like an army has group of several battalions, the defensive mechanism of the body called as the immune system is composed of several organs, each performing a significant role contributing to the overall protection of the body from pathogens. Based on the type of activity the organs provide they are classified into primary lymphoid organs, secondary lymphoid organs and tertiary lymphoid organs. The organs with specific role grouped under these categories are collectively called as the organs of the immune system.

The main function of the primary lymphoid organs is to develop matured lymphocytes and the secondary lymphoid organs acts as the ground for lymphocyte – antigen interaction. In an event of inflammatory reaction, the role of the tertiary lymphoid organ is to import the lymphoid cells.

Primary Lymphoid Organs: Bone marrow and thymus are the primary lymphoid organs acts as the sites for lymphocyte maturation. Lymphocytes are the major cells of immune system composed and classified into B-cells, T-cells and null cells based on their function. Bone marrow houses B-cell maturation whereas the T-cells mature in the thymus. The primary lymphoid organs outgrows in the fetal stage itself from the junction of ectoderm and endoderm or from the endoderm. The stem cells of the bone marrow are the source for all the cells involved in immune response. The soft tissue of the bone, the bone marrow is composed of two compartments called as the Haemopoietic compartment and the vascular compartment. The former enclosed by layer of reticular cells possess the precursors of all the blood cells, clusters of lymphocytes and macrophages and the later, the vascular compartment is lined by endothelial cells and crossed by reticular cells and macrophages.

Thymus, the flat bilobed structure located in the thoraxic cavity acts as the site for T-cell maturation. The bilobed structure has two sections called as the cortex and the medulla. Cortex, the outer compartment is crowded with Thymocytes, the immature T-cells and the medulla region is thinly populated with T-cells. The proliferation and maturation of thymocytes happens in the cortex region following which they pass through the medulla region for further maturation before leaving the thymus. The alpha and beta thymosine, thymopoietin, thymulin and thymostimulin are the hormonal factors of the thymus participating in the differentiation and maturation of the thymocytes. The Bursa of Fabricius is the bone marrow equivalent primary lymphoid organ present in birds.

Secondary Lymphoid organs: The secondary lymphoid organs develop from the cells of mesoderm in the later stages of fetal life. Antigenic response stimulates the growth of the secondary lymphoid organs. These organs are rich sources of B cells, T cells, macrophages and dendritic cells. The lymph node, spleen and the mucosa associated lymphoid tissue are the secondary lymphoid organs.

The bean or round shaped lymph nodes with reticular network packed with lymphocytes, macrophages and dendritic cells are the first tissues to encounter antigens entering the system. The morphological division of the lymph node has three sections namely the outer layer cortex followed by a second layer paracortex and the inner area medulla. Cortex is rich in B cells, macrophages and dendritic cells. The paracortex is populated with T cells and dendritic cells and the medulla region is sparsely populated with lymphocytes mainly B cells and fewer amount of macrophages. The fate of the antigen depends upon whether the person is already exposed to the antigen or not. An antigen entering the system for the first time is carried by the lymph to the lymph node, where it enters the medulla region and gets phagocytosed by the macrophages present in that region. The phagocytosed antigen travels to the paracortex region where they are encountered by the T cells which migrate to the cortex layer triggering the B cell division and the divided B cells (Antibody producing cells) travel back to the medulla and released into the efferent lymph of the lymph node allowing it to pass through the other lymph nodes. The entire process of antibody production in response to an antigen takes several days and if the person is exposed for the second time to the same antigen, the foreign molecules get trapped by the antibody coated dendritic cells present in the cortex region of the lymph node.

Spleen functions by filtering blood resulting in the removal of antigens and the old blood cells. Spleen is considered as the major site for antibody production and effector T cells. Spleen has two compartments called as the red pulp and the white pulp. Red pulp region forms and stores the red cells and also involves in antigen trapping whereas the white pulp, rich in lymphocytes is the site for immune response. Mucosa Associated lymphoid tissue (MALT) is the secondary lymphoid organ governing the mucosal lining of the digestive system, respiratory system and the urogenital system from infections.

The cutaneous associated lymphoid tissue is the tertiary lymphoid organ which is the skin. The external epidermal layer of the skin is composed of Keratinocytes, the epithelial cells secretes biologically active substance called as Cytokine which actively participate in local inflammatory reaction.