Lymphoid system Spleen

Thymus Is a central lymphoid organ situated in the mediastinum ,the thymus has dual embryonic origin mesodermal and endodermal origin .

The cortex is composed of an extensive population of T cell precursors (also called thymocytes ) ,dispersed epithelial reticular cells and macrophage in the cortex The developing T cells arise from CFU-Ls, which originate in bone marrow. As development proceeds in the thymus, the cells derived from CFU-Ls pass through a series of developmental stages that are reflected by their expression of different CD molecules

Epithelioreticular cells Six types of epithelioreticular cells are recognized on the basis of function: Three types in the cortex and three types in the medulla: 1- Type I Act as a barrier that isolates developing T cells from the connective tissue of the organ—that is, capsule, trabeculae, and perivascular connective tissue. 2- Type II Type II cells compartmentalize the cortex into isolated areas for the developing T cells

3- Type III create a functional barrier—in this case, between the cortex and medulla 4- Type IV In cooperation with type III cells, they create the barrier at the corticomedullary junction. 5- Type V They are located throughout the medulla. Processes of adjacent cells are joined by desmosomes to provide the cellular framework of the medulla and to compartmentalize groups of lymphocytes 6- Type VI They form thymic (Hassall’s) corpuscles, it is thought that thymic corpuscles produce interleukins (IL-4 and IL-7)

Blood vessels pass from the trabeculae to enter the parenchyma of the thymus. Typically, the blood vessels enter the medulla from the deeper parts of the trabeculae and carry a sheath of connective tissue along with them

This barrier is present only in the cortex and it is formed by the following layers: 1-Blood capillary wall (endothelium and basal lamina) 2- Connective tissue with macrophages 3-The basal lamina of the epithelial reticular cells and 4- The cytoplasm of epithelial reticular cells.

T cell education

Development and involution of the thymus : The thymus shows its maximum development immediately after birth and undergoes involution after puberty, in which the lymphoid tissue is replaced by adipose tissue. Involution begins in the cortical areas Both reticular cells and Hassall's corpuscles are resistant to involution The thymus never disappear completely, it is still present even in very old people.

Secretion of the thymus: The thymus produces several proteins that act as growth factors to stimulate the proliferation and differentiation of T lymphocytes.

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Spleen Is the largest lymphoid organ in the body , is important defense against Ags that reach blood circulation ,and it is the site of destruction of aged RBCs and production of activated lymphocytes , so spleen is an important filter and antibody forming organ .

General structure : Splenic pulp : The splenic pulp has two components the white and red pulp , these names derived from the fact that on the surface of a cut of spleen ,white spots are observed (lymphatic nodule ).

White pulp

Red pulp The red pulp is composed from splenic cords and sinusoids , the splenic cords contain a network of reticular cells supported by reticular fibers , it contain also T and B cells , macrophages , plasma cells and many blood cells . The splenic cords are separated by irregularly shaped wide sinusoids . elongated endothelial cells line the sinusoids , these cells are enveloped in reticular fibers set in transverse direction .

Closed and open theories of blood supply of the spleen The manner in which the blood flow from the arterial capillaries into the sinusoids has not completely understood. some investigators suggest that that the capillaries open directly into the sinusoid forming the (closed circulation) in which the blood remain inside the vessels . other suggest that prolongation of penicillar arteries open into the splenic cords and the blood pass through the spaces between the endothelial cells to reach the sinusoids (open circulation ) .

Functions of the spleen *Phagocytosis and immunological defense : Spleen is able to filter , phagocytose , and mount immunological response against blood borne Ags . *Destruction of erythrocytes : Erythrocytes have an average life span of around 120 days ,after that they are destroyed by the spleen ,a reduction in their flexibility and changes in their membrane seem to be the signal for their destruction . *Remove aged platelets .

Lymph nodes Lymph nodes are distributed throughout the body along the course of lymphatic vessels .Lymph nodes constitute a series of in-line filter that are important against the microorganism and spread of tumor cells . All lymph ,derived from tissue fluid is filtered by at least one LN

The most common cells of LN are lymphocytes , macrophages and APCs ,plasma cells and reticular cells ;follicular dendritic cells .

The LN has two regions :cortex and medulla situated beneath the capsule ,consisting of the following components : 1-lymphoid nodules ( mainly B cell) . 2-other immunogenic cells ( T cells ,APCs ,MQs ) 3-subcapsular sinus is situated beneath the capsule has loose lymphoid tissue . 4-intermediate or radial sinuses

Medulla : The medullary cords :are branched cordlike extensions of dense lymphoid tissue ,they contain B cells ,MQ and plasma cells . The medullary sinuses :the medullary cords are separated by dilated spaces bridged by reticular cells and fibers .

Lymph circulation

Role of lymph nodes in immune response The lymph that arrives at a lymph node may contain Ags ,the Antigens that not has been phagocytosed may be internalized by APCs of the lymph node .All the Ags have the opportunity to be presented to B and T cells to initiate immune response .

Recirculation of lymphocytes : Lymphocytes that leave the lymph nodes by efferent vessels eventually reach the blood stream , they may return to a lymph node by a process called (homing )by crossing the wall of specific blood vessels , the high endothelial venules present in the lymph node , appendix , tonsil and peyer's patches . These venules have an unusual endothelial lining of tall cuboidal cells .

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