ORGANS OF THE IMMUNE SYSTEM

BY MRS. N .MAKANDI ORGANS OF THE IMMUNE SYSTEM Major organs of the immune system are bone marrow, thymus, spleen and lymph nodes. These organs produce lymphocytes required to destroy bacteria, virus, tumor cells, etc.

NB// The function of the immune system is protecting the body from parasitic, bacterial, viral, fungal infections and from the growth of tumor cells.

• Organs of the immune system make cells that either contribute in the immune response or act as sites for the immune function.

There are two groups of immune system organs. • Primary (central) organs where immature lymphocytes develop – Thymus – Bone marrow • Secondary (peripheral) organs --tissues where antigen is localized so that it can be effectively exposed to mature lymphocytes – Lymph nodes – Spleen – MALT (Mucosal-Associated Lymphoid Tissue) • GALT (Gut-Associated Lymphoid Tissue) • BALT (Bronchial/Tracheal-Associated Lymphoid Tissue) • NALT (Nose-Associated Lymphoid Tissue) • VALT (Vulvovaginal-Associated Lymphoid Tissue) Primary (central) lymphoid organs Bone marrow • All the cells of the human immune system are formed in the bone marrow, found within the bones, by a process called hematopoiesis.

• The process of hematopoiesis involves differentiation of bone-marrow derived stem cells either into mature cells of the immune system or precursor of cells which move out of the bone marrow and continue their maturation elsewhere.

• The bone marrow is responsible for the production of important immune system cells like B cells, granulocytes, natural killer cells and immature thymocytes. It also produces red blood cells and platelets • Bone marrow is the site of B cell maturation.

• The site of B cell maturation in birds is the bursa of Fabricius, after which B cells are named.

• The tissue of bone marrow where leukocytes, red blood cells, and platelets develop (i.e., the site of hematopoiesis) is known as myeloid tissue. Thymus • The thymus is the site of T cell maturation.

• T cells become immunocompetent here; that is, they develop their ability to mount an effective immune response against foreign invaders without attacking the host's own tissues.

• The main function of the thymus gland is to produce mature T cells.

• The immature cells produced at the bone marrow, migrate and come into the thymus, where the maturation process takes place.

• This maturation process is a remarkable one as it allows only the beneficial T cells to be released into the bloodstream. T cells that evoke a detrimental autoimmune response get eliminated. • • The thymus lies just above the heart in the mediastinum. It is largest in childhood, and it begins to shrink significantly as a person ages. • The organ itself contains two lobes, and each lobe contains numerous lobules, separated from each other by connective tissue septa known as trabeculae.

• Each lobule is separated into an inner medulla (with few immature thymocytes) and an outer cortex (with large numbers of immature thymocytes).

• In the thymus, the many different T cells are exposed to MHC/Ag and MHC/self-Ag. If they do not react to MHC/Ag, they are destroyed due to their ineffectiveness (positive selection). i.e T cells that can recognize self MHC's are selected for proliferation, and those T cells that do not recognize self MHC die.

• On the other hand, if they do react to MHC/self-AG, they are destroyed in order to stop them from causing autoimmune responses against the body's own tissue(negative selection). i.e T cells that are strongly activated by self MHC plus self peptides need to be eliminated in the thymus. If they escape this elimination, they may subsequently react against self antigens, and cause Autoimmune disease.

• • Only 1 out of 20 immature thymocytes will pass successfully through this vetting process and become functional T cells.

• Dendritic cells, macrophages, and epithelial cells are interspersed throughout both the medulla and cortex; special epithelial nurse cells surround clusters of thymocytes in the cortex. Secondary (peripheral) lymphoid organs

spleen • The spleen acts as a site of hematopoiesis during the second and third trimesters of development, before the long bones have fully developed.

• In the adult, the spleen acts as a site for breakdown of dying red blood cells (lifespan 120 days). For this reason, enlargement of the spleen (splenomegaly) can occur in sickle cell anemia or in certain infections.

• White pulp, near the arteriolar entry points into the spleen, is where lymphocytes reside and are degraded.

• The central red pulp is the site of RBCs breakdown.

• The white pulp region has a central part, with the T cells residing in the PALS, or PeriArteriolar Lymphoid Sheath and a B cell ring (or corona) surrounding the PALS.

• It acts as an immunologic filter of the blood and entraps foreign materials, that is antigens from the bloodstream passing through the spleen.

• When the macrophages and dendritic cells bring antigens to the spleen via the bloodstream, the B cells in the spleen get activated and produce large levels of antibodies. Thus, the spleen can also be known as the immunologic conference center. Lymph Nodes

• Just like the way the spleen filters the blood, lymph nodes filter the interstitial fluid present between the cells of the human body.

• Lymph nodes are situated throughout the lymphatic system of the body and are nothing but aggregations of tissues.

• Lymph nodes are made up of mostly B cells, T cells, macrophages and dendritic cells.

• They act as immunologic filters and drain the lymph from most of the body tissues and filter out the antigens present in them, before allowing the lymph to return to circulation. • The paracortical areas of the nodes contain T cells

• The central areas contain germinal centers, where B cells are contained.

• APCs and antigen are sent from the tissue into the lymphatics, eventually reaching the lymph nodes where they can be exposed to the T and B cell populations. This allows a faster response, as the many combinations of T and B cell specificities are able to reside in several locations throughout the body (the lymph nodes) rather than relying on random meetings of antigen and lymphocytes throughout the tissues themselves. Mucosal associated lymphoid tissue • Mucosa systems protects mucosal surfaces.

• It is the site of first encounter of immune cells with antigens entering through mucosal surfaces.

• Thus lymphoid tissues are associated with surfaces lining the intestinal tract (gut associated lymphoid tissue), the respiratory tract (bronchus asociated lymphoid tissue), and the genitourinary tract (areas exposed to numerous antigens).

• The major effector mechanism of mucosal surfaces is secretory IgA antibody (SIgA) , actively transported through the mucosal epithelial cells to the lumen of the tracts. • Mucosa associate lymphoid tissue contains both B and T cells as well as phaghocytes.

• It forms a separate interconnected secretory system comprising of IgA or IgE producing cells. • In the alimentary tract, antigen pasing across specialised apithelial cells enters the payer’s patches and stimulate the lymphocytes.

• The stimulated lymphocytes are drained into the lymph which ultimately reach blood stream via thoracic duct. • They pass into the lamina propria from the blood stream, where the become IgA forming cells.

• B cells predominate in gut associated lymphoid tissue.