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ORGANS OF THE IMMUNE SYSTEM
(LYMPHOID ORGANS)
Cells involved in the immune reaction are organized into tissues and organs which are collectively referred to as the lymphoid system. This system performs the major function of protecting the body against various microorganisms that enter the body. Lymphoid organs are those organs where the maturation and proliferation of lymphocytes take place. There are two types of lymphoid organs: a) Primary lymphoid organs. b) Secondary lymphoid organs LYMPHOID ORGANS Primary lymphoid organs secondary lymphoid organs Thymus, Bone marrow Lymph node, spleen
A) PRIMARY LYMPHOID ORGANS:
Primary lymphatic organs are where lymphocytes are formed and mature. They provide an environment for stem cells to divide and mature into B-cells and T- cells. There are two primary lymphoid organs: The thymus and the Bone marrow. THYMUS: It is a site of T-cells development and maturation. It is a flat bi-lobed organ situated above the heart. Each lobe is surrounded by a fibrous capsule and divided into lobules, which are separated from each other by strands of connective tissue called Trabeculae. The thymus is organized into 2 regions cortex and medulla The outer compartment cortex is densely packed with immature T-cells called thymocytes.
The inner compartment or medulla is populated with thymocytes. There are four types of cells found in the thymus. They are Thymocytes, dendritic cells, epithelial cells, and macrophages. Both cortex and medulla of the thymus are crisscrossed by a 3D dimensional stromal- cell network. Nurse cells are present in the cortex which internalize 50 thymocytes. These cells help in the differentiation of T-cells. Epithelial cells of the thymus are known to produce some hormones namely: thymulin, Alpha- thyrosin in, beta-thymosin, thymopoietin A central part of the thymus is Hassel's corpuscles. Functions: Thymus produces T-Lymphocytes. Out of four cell types, dendritic cells, epithelial cells, and macrophages act in a combined manner, as a framework to assist in thymocyte maturation. It helps in graft rejection. It helps in cell-mediated immunity. . CHANGES IN THE SIZE OF THE THYMUS: The average weight of the thymus in an infant is 70 grams and for adults, it will reduce to 30 grams. A thymic function is known to decline with the age. The thymus reaches its maximum size at puberty(10-12) and then atrophies with a decrease in both cortical and medullary cells. As the age increases there is a loss in mass which is accomplished by the decline in T- cell output.
Red marrow is mainly found in flat bones such as the pelvis, sternum, cranium, ribs, vertebrates, and the epiphyseal end of long bones like the femur. The red bone marrow contains blood vessels and capillaries. At birth, all bone marrow is red with increasing age, red marrow is converted into yellow marrow. In adults, around half of the marrow is yellow, the remaining red. ii) YELLOW MARROW: It is the vascular region of the circulatory system that supplies nutrients and removes waste from the actively growing blood cells. It consists of fat cells Yellow marrow is found in the interior or middle portion of the long bones. The yellow marrow contains blood vessels. iii) STROMA: Stroma is the tissue of bone marrow, that is not directly involved in hemopoiesis. Yellow bone marrow makes up the majority of the stroma. Some red marrow also constitutes the stroma. The following cells constitute stroma fibroblast Macrophages, adipocytes, osteoblast, endothelial cells, mesenchymal stem cells. The stroma is directly involved in hemopoiesis. It provides a hemopoietic microenvironment. FUNCTIONS OF BONE MARROW : Bone marrow functions both as primary and secondary lymphoid organs, responding rapidly to antigens. Bone marrow gives long-lasting massive production of antibodies to the antigen. Stem cells arise from the bone marrow from hemopoietic stem cells of bone marrow. B-lymphocytes develop from the bone marrow. They secrete a large number of antibodies, so bone marrow is a source of antibody synthesis.
B) SECONDARY LYMPHOID ORGANS:
Besides the primary lymphoid organs, some other lymphoid organs are referred to as secondary lymphoid organs. Secondary lymphoid tissues are also where lymphocytes are activated. These include lymph nodes, spleen, and tonsils. The lymph node and spleen are the most important and highly organized secondary lymphoid organs.
Less organized lymphoid tissue is associated with the mucosal surface of the body and is referred to as MALT of the gut, respiratory and urinogenital tract. MALT includes – Peyer patches(in the small intestine), tonsils, and appendix. Lymphoid tissues activated with the elementary canal are commonly referred to as GALT(gut-associated lymphoid tissue). LYMPH NODE : Lymphatic system transfer the lymph, fluid containing infection-fighting WBC throughout the body. The lymphatic system is a network of tissues or organs. The lymph node is a complex, cellular, spherical, or ovoid structure present along the lymphatic ducts. Each node is made of three regions namely an outer cortex, a middle, a paracortex, and an inner medulla. The cortex is covered by capsule follicular cells differentiated by trabaculae. Paracortex contains lymphocytes, macrophages, and follicular dendritic cells. The bulk of lymphoid cells is present in the cortex. These cells move together to form nodules or primary follicles within this primary follicle, secondary follicles will arise along with germinal centers. Paracortex contains T-lymphocytes and interdigitating dendritic cells that act as antigen-presenting cells and thus express class II MHC molecules. The innermost layer of the lymph node is composed of lymphocytes, forming interconnecting strands in the medulla, called the medullary cord. Again these medullary cords surround the medullary sinuses which have plasma cells and some macrophages. FUNCTIONS: Lymph nodes are the centers of phagocytosis.
Spleen is said to be the graveyard of RBCs. Recycle RBC and collects iron. During the embryonic stage, before bone marrow formation, the production of RBC is by the spleen.
