Chapter 20: Lymphatic System

Lymphatic System: Overview Lymphatic vessels Lymphoid tissues and organs Returns interstitial fluid and leaked plasma proteins back to the blood

Lymphatic Vessels Capillaries → collecting vessels → nodes → trunks → ducts

Lymph Transport No pumping organ Vessels are low-pressure conduits Uses the same methods as veins to propel lymph:  Pulsations of nearby arteries and muscles  Respiratory pump  valves

Lymphoid Cells Lymphocytes □ T cells □ B cells Macrophages Dendritic cells Reticular cells

Lymphoid Tissue Proliferation site for lymphocytes Surveillance point for immune cells Reticular tissue (except in thymus) Diffuse lymphatic tissue □Peyer's patches, MALT, appendix Lymphatic follicles (nodules)

Lymph Nodes Principal lymphoid organs Filter lymph and activate immune system Embedded in connective tissue Aggregations of nodes  Inguinal, axillary, and cervical regions of the body

Structure of a Lymph Node Cortex □Germinal centers Medulla □ Medullary cords Lymph Sinuses

Spleen Fibrous capsule, trabeculae Lymphocytes and macrophages Functions:  Site of lymphocyte proliferation  Immune surveillance and response  Cleanses the blood

Figure 20.6a, b

Thymus T cell maturation Macrophages No B cells (no follicles) Decays with age Size of the thymus varies with age Epithelial cells rather than reticular tissue

Tonsils Follicles with germinal centers Not fully encapsulated Epithelial tissue forming blind-ended crypts Crypts trap and destroy bacteria and particulate matter

Aggregates of Lymphoid Follicles Peyer’s patches – wall of small intestine  Similar structures are found in the appendix MALT – mucosal associated lymphoid tissue