Lymphoid organs

LYMPHOID ORGANS DEFINITION:The organs concerned with the production, maturation and proliferation of lympho­cytes are called as lymphoid organs.Types of Lymphoid organs�Primary or central lymphoid organs
and
Secondary or peripheral lymphoid organs, Thymus, THYMUS-CORTEX,CORTICOMEDULLARY JUNCTION,Thymus- Medulla,Functions of Thymus,Thymectomy,Bone marrow (BM),FUNCTIONS OF BONE MARROW,BM functions as both primary and secondary lymphoid organs.

1. LYMPHOID ORGANS Dr. R. RENUKA ASSOCIATE PROFESSOR OF BIOCHEMISTRY V.V.VANNIAPERUMAL COLLEGE FOR WOMEN VIRUDHUNAGAR , TAMILNADU.

2. DEFINITION The organs concerned with the production, maturation and proliferation of lympho­cytes are called as lymphoid organs.

3. Types of Lymphoid organs There are two types of lymphoid organs: Primary or central lymphoid organs and Secondary or peripheral lymphoid organs.

4. 1°(central) lymphoid organs These are the major site for the formation of lymphocytes. Here, the T lymphocytes and В lymphocytes mature into immunocompetent cells by acquiring their antigen-specific receptors (in the absence of any antigenic stimulation). After maturation, the lymphocytes migrate to secondary lymphoid organs through blood and lymph. Primary lymphoid organs include thymus and bone marrow . T cells mature in thymus and B cells in bone marrow.

5. 2° or peripheral lymphoid organ These organs are those where antigen is concentrated and allowed to interact with immune cells for proper immune response. These include spleen, lymph node and MALT that includes NALT, GALT, BALT and genitourinary system. In addition, there are 3° lymphoid organ that consists of loose aggregates of lymphocytes often found in connective tissue. The most important 3° lymphoid tissues are the cutaneous- associated lymphoid tissues.

6. Thymus Thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. It resembles a thyme leaf and hence the name. It is situated below the thyroid gland and above the heart in the thorax region as a derivative of pharynx . It begins its development on the 6 th week of gestation. It is quite large in size at the time of birth but keeps reducing with age. It is a bilobed structure and the t wo lobes are covered by a fibrous capsule . Each lobe is organised into lobules which are separated from one another by septae called as trabeculae .

7. THYMUS-CORTEX Each lobule is organized into 2 compartments - outer cortex and inner medulla criss -crossed by the network of stroma. The cortex consists of immature T lymphocytes (thymocytes), reticular epithelial cells and macrophages. As the T cells mature, they move from cortex to medulla. The reticular cells form a three dimensional network and in the meshes of which the lymphocytes are found. These reticular cells are also called as ‘nurse cells’. In thymus, thymocytes undergo rapid proliferation. Rapid cell division is coupled with a high rate of cell death so that finally a very small subset of thymocytes migrate from cortex to medulla after thymic education . On the way to medulla, thymocytes begins to express receptors for Ags and surface markers.

8. THYMUS - CORTICOMEDULLARY JUNCTION It is the site where the selection of right T-cells occurs. T cells undergo thymic education through positive and negative selection. T cells either self- reactive or do not recognize foreign Ag displayed on self MHC are deleted (negative selection). Only the T cells whose receptor recognizes a foreign Ag plus self- MHC molecule are allowed to mature and enter the medulla (positive selection).

9. Cross section of part of thymus

10. Thymus- Medulla The medulla consists of vascular structure, more reticular epithelial cells and scattered mature T lymphocytes. There are also some interdigitating cells associated with the epithelial network and these cells are rich in MHC class II antigens. Hassall’s corpuscles - the whorls of epithelial cells around a central core of degenerating epithelial cells with unknown functions are also present.

11. Thymus… contd … The T lymphocytes are mature but nonfunctional or naive when they are inside the thymus, as they do not come in contact with the antigens. This is mainly because of the presence of epithelial barrier which prevents the antigens of the blood to contact the lymphocytes of the thymus. These non-functional T lymphocytes travel through blood and lymph circulation and finally reach the thymus dependent areas of the secondary lymphoid organs (spleen) and get colonized there. In the secondary lymphoid organs, these T cells become functional by antigenic stimulation.

12. Functions of Thymus Thymus is the site for T cell maturation and proliferation. It brings about cell mediated immunity. It is responsible for graft rejection.

13. Thymectomy Removal of thymus in mice results in Impairment of cell mediated immunity drastic change in allograft rejection delayed type hypersensitivity reaction Wasting syndrome (hunched appearance, ruffled fur and diarrhoea.) Immunodeficiency In humans, Congenital Birth Defect- Di George Syndrome (delays in learning to walk or talk and other developmental and learning delays).

14. Bone marrow (BM) BM is a major haematopoietic cellular connective tissue within the bone that produces all immune cells and delivers them into the circulation. There are 3 types of bone marrow tissues: Red Marrow White Marrow Stroma BM is richly supplied with blood vessels and capillaries but it lacks lymphatic vessels.

15. Bone marrow… contd … Red marrow -red because of the presence of RBC and its precursors. Site of active haematopoiesis found in the flat bones. At birth all bone marrow are red. With increasing age almost half of it is converted into yellow marrow. Yellow marrow - yellow because of the presence of fat cells found in the interior of long bones. Reduced haematopoiesis . These two forms are interconvertible as demands change. Stroma - tissue of BM provides microenvironment for haematopoiesis . Cytokines and growth factors are produced. Supply ion to RBC.

16. FUNCTIONS OF BONE MARROW Site of stem (blood) cell formation. Functions both as primary and secondary lymphoid organs B cells origin and mature Selection process occurs that deletes those B cells which are self – reactive Mature B cells exit via vascular sinuses and enter the blood stream Some of the lymphoid stem cells migrate to thymus for their maturation into T cells. Cytokines and growth factors are secreted by stroma of bone marrow. Bone marrow is also a source of antibody synthesis

17. BM functions as both primary and secondary lymphoid organs Bone marrow functions as primary lymphoid organ as the B cells are developed and differentiated into immunocompetent B cells in it. In mammals, few days after secondary response, activated memory B cells migrate into bone marrow where they mature into plasma cells. By producing specific antibodies to antigens, the bone marrow acts as a secondary lymphoid organ.