CH. 21: The Immune System

An extensive network of one-way lymphatic vessels, lymphatic tissues and organs

Return of excess fluid (3 liters/day) Defense against disease using lymphocytes at the lymph nodes Transportation of absorbed fats by lymphatics moving filtered proteins to the venous fluids (too big for capillaries) Return of filtered proteins because filtered proteins cannot return back into the capillaries so they are returned through the venous fluids via lymphatics No unique functions of its own (has to work with other systems)

An interstitial fluid that moves into lymphatic capillaries due to pressure gradient during the capillary exchange -15% of fluid does not get reabsorbed back into the capillaries -Blood is not returned as much as it was filtered out, so it goes through the lymphatic vessels Components: -Water & dissolved solutes -Few proteins -Foreign materials (cellular debris, pathogens, metastasized cancer cells)

Are initial lymphatics located at the terminal end of the lymphatic system Components: -Blind-ended -Intertwined with the capillary bed -Interspersed throughout areolar connective tissue except in red bone marrow and CNS -Absent in avascular tissue

Larger than capillaries Lack of a basement membrane Possess free-edged overlapping endothelial cells serving as one-way mini valves to allow fluid into the capillary Anchoring filaments hold the endothelial cells in place to prevent lymphatics from collapsing due to excessive interstitial fluid hydrostatic pressure (HPif)

The pressure drives function where interstitial fluid hydrostatic pressure (HPif) forces fluid between endothelial cells, and internal hydrostatic pressure forces endothelial cells together preventing fluid outflow to ECF Escaped proteins, cellular debris, & pathogens are shunted to lymphatics because the openings are larger than capillary pores (following the direction of bulk flow)

Vessels that are formed by the convergence of the lymphatic capillaries Superficial lymphatics are adjacent to superficial veins, and deep lymphatic vessels are adjacent to both deep arteries and veins Posses all 3 vessel tunics and one-way valves Lacks a pump so it relies on the skeletal and respiratory pump to drive pulsatile movement of blood in nearby arteries. The rhythmic contraction of lymphatic smooth muscle also helps with lymph flow

They form the union of lymphatic vessels and are named for the region of the body from which they collect lymph Paired Trunks (right & left) -Jugular: head and neck -Subclavian: upper limbs, breasts, & superficial thoracic wall -Bronchomediastinal: deep thoracic structures -Lumbar: lower limbs, pelvis, and abdominopelvic wall Unpaired Trunks -Intestinal: most abdominal structures

They are where lymph empties into the venous circulation Consists of right lymphatic duct and thoracic duct

Larger of the two ducts and is responsible for the rest of the body extending from the left subclavian and jugular veins and consist of the cisterna chyli at the end of the duct Cisterna Chyli -Receives from vessels that drain the small intestine, intestinal trunk, and from lumbar trunk -Lipid-rich lymph (chyle)

Involved in the formation and maturation of lymphocytes Includes the red bone marrow and thymus

House lymphocytes and other immune cells following their formation Provide the site where an immune response is initiated Includes the lymph nodes, spleen, tonsils, lymphatic nodules, and MALT

They are found in between trabeculae in selected portions of spongy bone In adults, it includes flat bones of the skull, vertebrae, ribs, sternum, ossa coxae, and proximal epiphysis of the humerus & femur Responsible for hematopoiesis: -T-cells: must migrate from bone marrow to thymus to complete maturation -B-cells: mature in the bone marrow

Composition: -Lobules are composed primarily of the epithelium to secrete hormones (thymopoietin & thymosins) that participate in T-cell maturation -Infiltrated with T-cells at various stages of maturation The most important function is early in life and continues to grow until puberty and regresses into adulthood (thymic tissue is replaced by adipose tissue)

Two fused lobes surrounded by a connective tissue capsule Extensions of the capsule (trabeculae) subdivide the lobes into lobules Lobules are arranged by the outer region (cortex) and inner region (medulla) Blood-thymus barrier prevents entry of bloodborne pathogens into the cortex and premature activation of the immature T-cell

Macrophages - phagocytize foreign substances Dendritic Cells - capture antigens & bring them to the lymph nodes Immune cells - are enmeshed within a reticular extracellular matrix

Organized into either an organ or an aggregate of lymphatic nodules in the presence/absence of a dense irregular CT capsule Complete CT capsule = organ -Lymph nodes and spleen Incomplete or absent capsule = other lymphatic structure -Tonsils, MALT, and diffuse lymphatic nodules

Clusters that receive lymph from selected body regions or individually throughout the body -Most are embedded in CT and not readily observed -Large clusters are found near the body surface in the axillary, inguinal, and cervical regions Functions to filter the lymph before it returns to the bloodstream -Macrophages in the node destroy microorganisms and other cellular debris to prevent them from entering the bloodstream -Monitor for the presence of antigens and mount attacks against them

Capsule Outer cortex Inner medulla Cortical and Medullary sinuses

It is a dense fibrous enclosure made of trabeculae Trabeculae -connective tissue strands that extend from the capsule to divide the node into compartments

It is the activation and proliferation site of lymphocytes that is divided into the lymphatic nodules Lymphatic Nodule -Composed of reticular fibers -Activation/proliferation site for B-cells in inner germinal centers -Outer mantle zone contains T-cells, macrophages, and dendritic cells

It is the center of most portion of the node Consists of medullary cords -CT extension from the lymphoid tissue of the cortex -Supports B-cells, T-cells, and macrophages

The lymph capillary system allows the lymph to be exposed to the cells in the node that is lined with macrophages Includes cortical and medullary sinuses

• Afferent Lymphatic Vessels -Numerous • Sinuses (cortical and medullary) -Macrophages remove foreign debris -Lymphocytes come into contact with foreign debris and become activated -Cortical Sinus initiates B-cell activation and proliferation -Medullary Sinus is where B-cells & T-cells enter the circulation to sites of infection • Efferent Lymphatic Vessel -Only one per many afferent vessels because lymph flows slow down in the node to allow the cells within the node to be exposed longer to the lymph and perform their protective function -Originates from the hilum Lymph nodes move through a series of nodes (in a cluster) and the same lymph is repeatedly screened for unwanted materials