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

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

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

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. MAL T (Mucosa-Associated Lymphatic Tissue) The appendix vermiform is other example of MALT. It walls contain a mass of fused lymphoid nodules.

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

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

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

Lymph nodes Lymph vessel Lymph nodes Hilum Medullary sinus Outer cortex (B cells) Afferent vessel Medullary cord (B cells and plasma cells) Capsule Dense connective tissue Deep cortex (T cells) Subcapsular space Medulla Trabeculae Efferent vessel Lymph node artery and vein Lymph nodes are shaped like kidney beans. Cortex

Capillary Capsule Subcapsular space Outer cortex Germinal center Dividing B cell Dendritic cells Nuclei of B cells

Lymph nodes Hilum Medullary sinus Outer cortex (B cells) Afferent vessel Medullary cord (B cells and plasma cells) Capsule Dense connective tissue Deep cortex (T cells) Subcapsular space Medulla Trabeculae Efferent vessel Lymph node artery and vein Lymph Flow Cortex Dendritic cells and macrophages that start the immune response The medulla contains B cells and plasma cells responsible for the production of antibodies (humoral immunity).

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).

The Thymus The thymus is located in the mediastinum. It atrophies after puberty. The thymus is divided into two thymic lobes. Right lobe Left lobe Septa Lobule Septa divide lobes into smaller lobules. Thymus

Lobule The thymus gland LM  50 Cortex Septa Medulla

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

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

The spleen LM  50 White pulp of splenic nodule Capsule Red pulp Trabecular artery Central artery in splenic nodule White pulp is dominated by lymphocytes. Red pulp contains a large number of red blood cells. 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. Histology of the Spleen