Lesson # 9

1. Lesson # 9 Chapter 22 The Lymphoid System-2 Objectives: 1- To list the major lymph organs and the functions of each.

2. The Lymphatic System Lymphatic Vessels Diffuse Lymphoid Tissues Encapsulated Lymphoid Organs Lymph It is the recovered fluid. Lymph is usually a clear colorless fluid, similar to blood plasma but low in protein. - Lymphatic capillaries Aggregates of lymphocytes in the connective tissue of mucous membrane and various organs. They have well –defined anatomical sites and at least partial connective tissue capsules. - Lymphatic collecting vessels Mucosa-associated lymphatic tissue (MALT) - Lymphatic trunks 1- Lymph nodes - Collecting duct 1- Peyer patches in the distal portion of the small intestine. 2- Thymus 3- Spleen 2- Vermiform appendix 3- Tonsils (palatine tonsils and pharyngeal tonsils or adenoids).

3. Lymphoid Tissues Lymphoid tissues are areolar connective tissues with concentrations of lymphocytes. Germinal center When lymphocytes are densely packed they form lymphoid or lymphatic nodules or follicles. The lymphatic nodules or follicles have no fibrous capsule surrounding them. Lymphatic nodule or follicle It contains dividing lymphocytes.

4. MALT (Mucosa-Associated Lymphatic Tissue) It is the collection of lymphoid tissues that protect the epithelia of the respiratory, digestive, urinary, and reproductive systems. • Clusters of lymphoid nodules deep to the epithelial lining of the intestine are known as Peyer’s patches. • The appendix vermiform is other example of MALT. It walls contain a mass of fused lymphoid nodules.

5. Tonsils Pharyngeal tonsil or adenoid (single) Palatinetonsils (two) Lingual tonsils (two) They are large concentration of lymphoid nodules in the walls of the pharynx. Functions • They guard against ingested or inhaled pathogens.

6. Lymph nodes Lymphoid Organs Spleen Thymus Lymphatic organs have well-defined anatomical sites and have connective tissue capsule that separates the lymphatic tissue from neighboring tissues. - Lymph nodes - Thymus - Spleen Lymph nodes They are the smallest and most numerous lymphatic organs (about 450 in typical young adult). They range in diameter from 1 mm to 25 mm (about 1 inch)

7. Lymph nodes are wide-spread but specially concentrated in some locations. Axillary lymph nodes Cervical lymph nodes Lumbar lymph nodes Pelvic lymph nodes Inguinal lymph nodes

8. Lymph nodes are shaped like kidney beans. Lymphnodes Lymphvessel Lymph nodeartery and vein Efferentvessel Hilum Lymph nodes Trabeculae Medullary sinus Cortex Medulla Subcapsularspace Outer cortex (B cells) Deep cortex(T cells) Afferentvessel Medullary cord(B cells andplasma cells) Capsule Dense connective tissue

9. Subcapsularspace Germinalcenter Outercortex Capsule DividingB cell Dendritic cells Nuclei of B cells Capillary

10. Lymph Flow Lymph nodeartery and vein Efferentvessel Hilum Lymph nodes Trabeculae Medullary sinus Cortex Medulla Dendritic cells and macrophages that start the immune response. Subcapsularspace Outer cortex (B cells) 6 7 5 1 2 3 4 Deep cortex(T cells) The medulla contains B cells and plasma cells responsible for the production of antibodies (humoral immunity). Afferentvessel Medullary cord(B cells andplasma cells) Capsule Dense connective tissue

11. Lymph Node Functions: • 1- They filter and purify the lymph before return to venous circulation (they remove 99% of cell debris, bacteria, viruses, fungi). • 2- They provide an early warning system against infection in peripheral tissue, responding before infections reach vital organs of trunk. • 3- The first step in immune response takes place in the lymph nodes (extracted antigens are “presented” to lymphocytes).

12. The Thymus • The thymus is located in the mediastinum. • It atrophies after puberty. • The thymus is divided into two thymic lobes. Thymus • Septa divide lobes into smaller lobules. Leftlobe Rightlobe Septa Lobule

13. Septa Lobule Cortex • Mature • lymphocytes Medulla • Dividing lymphocytes Lobule The thymus gland LM  50

14. Medulla Septa Cortex Lymphocytes Lobule Reticularcells Lobule Thymiccorpuscle The thymus gland LM  50 A thymic corpuscle LM  550 T lymphocytes divide in the cortex and mature while they migrate to the medulla. • Mature T cells leave thymus by medullary blood vessels. • The thymus secretes thymic hormones that stimulate T cell differentiation.

15. The Spleen It is the body’s largest lymphatic organ. Parietal peritoneum Visceral peritoneum Spleen Stomach Diaphragm Rib Gastrosplenicligament Liver Pancreas Gastric area Aorta Diaphragmatic surface SPLEEN Hilum Renal area Kidneys

17. Histology of the Spleen White pulp isdominated by lymphocytes. White pulp ofsplenic nodule Capsule Red pulp contains a large number of red blood cells. Red pulp Trabecularartery Central artery insplenic nodule The spleen LM  50 Functions: 1- Removal of abnormal and worn out blood cells. • 2- Storage of iron recycled from red blood cells. • 3- Initiation of immune responses by B cells and T cells.