BIOL 226 Chap 20: Lymphatic System and Immunity

• Visual Anatomy & Physiology, Third Edition presents Chapter 20 on the Lymphatic System and Immunity.

The Lymphatic System

• Includes cells, tissues, and organs that perform the roles of immunity and maintaining normal blood volume plus composition of interstitial fluid.
• Immunity is the ability to resist infection and disease, the immune system includes all cells and tissues involved.

Lymphatic System Overview

• Lymphocytes are primary cells that respond to pathogens, abnormal body cells, and toxins.
• Lymph is interstitial fluid that has entered a lymphatic vessel.
• Lymphatic vessels, also called lymphatics, begin in peripheral tissues and end at connections to veins.
• Lymphoid tissues and organs are scattered throughout the body, these are sites where lymphocytes are formed, mature, and are activated.
• Primary lymphoid tissues and organs include red bone marrow as well as the thymus gland.
• Secondary lymphoid tissues and organs are where lymphocytes are activated and cloned, these include lymph nodes, tonsils, MALT, appendix, and spleen.

Lymphatic Vessels

• Lymphatic vessels carry lymph from peripheral tissue to the venous system, and are found in close association with blood vessels.
• The network of these vessels begins with lymphatic capillaries, the smallest vessels.
• Lymphatic capillaries differ from blood capillaries, they are closed at one end, have larger diameters, and have thinner walls.
• Lymphatic capillaries lack a complete or have an absent basement membrane, and typically have a flattened or irregular outline in sectional view
• Lymphatic capillaries have overlapping endothelial cells, where the region of overlap acts as a one-way valve
• The overlapping endothelial cells allows entry of fluids, solutes including proteins, viruses, bacteria, and cell debris, but prevents return of these materials to the intercellular space.
• Lymph vessel structure describes that lymph flows into larger lymphatic vessels that lead to the body's trunk.
• Small to medium-sized lymphatic vessels contain valves that are located closely together and cause the vessel to bulge at each valve, making it resemble a string of pearls.
• Low pressure in lymphatic vessels, valves prevent backflow of lymph, contraction of surrounding skeletal muscles aids flow of lymph within the lymphatic vessels.
• Prominent lymphatic capillaries called lacteals are in the small intestine and transport lipids absorbed from the digestive tract.
• Areas lacking lymphatic capillaries are areas without a blood supply, such as the cornea of the eye.

Lymphatic Vessel Locations

• Superficial lymphatics are located in the subcutaneous layer deep to skin, in areolar tissues of mucous membranes, as well as serous membranes.
• Deep lymphatics accompany deep arteries and veins supplying skeletal muscles and other organs of the neck, limbs, trunk, and wall of visceral organs

Lymphatic Flow

• Superficial and deep lymphatics converge to form lymphatic trunks, these empty into either the thoracic duct or the right lymphatic duct.
• The thoracic duct collects lymph from the entire body inferior to the diaphragm, and the left side of the body superior to the diaphragm; it drains into the left subclavian vein.
• The right lymphatic duct collects lymph from the right side of the body superior to the diaphragm, and drains into the right subclavian vein.
• The right lymphatic duct is formed by the merging of the right jugular, subclavian, and bronchomediastinal trunks.
• The thoracic duct collects lymph from the left bronchomediastinal trunk, which drains into the left subclavian as well as jugular trunks.
• The cisterna chyli is an expanded sac-like chamber at the base of the thoracic duct that receives lymph from the inferior part of the abdomen, the pelvis, as well as the lower limbs Lymph is drained into the cysterna chyli by the lumbar and intestinal trunks.
• Lymphedema is caused by blocked lymphatic drainage, and the affected area becomes swollen and distended due to interstitial fluids accumulating.
• Lymphedema is seen most often in limbs and swelling may become permanent as connective tissue loses elasticity; stagnant interstitial fluids may accumulate toxins and pathogens and overwhelm local immune defenses.

Lymphocytes

• Lymphocytes make up 20-40% of circulating leukocytes
• Most lymphocytes are out in lymphatic tissues
• There are three classes of lymphocytes that circulate in blood: T cells, B cells and NK cells
• T cells consist approximately (~80%) of circulating lymphocytes and they are responsible for cell-mediated immunity
• B cells consist approximately (~10-15%) of circulating lymphocytes and they are responsible for antibody-mediated immunity
• NK cells consist approximately (5–10%) of circulating lymphocytes and have a key role in performing immune surveillance
• All classes of lymphocytes are sensitive to specific chemicals called antigens: most antigens are pathogens, foreign proteins, or abnormal substances.
• Antigens stimulate an immune response, which leads to destruction of the target compound or organism.

T cell Types

• Cytotoxic T cells attack foreign cells or body cells infected by viruses commonly involves direct contact, cytotoxic cells are the primary cells involved in production of cell-mediated immunity
• Helper T cells stimulate activation and function of T cells and B cells by activating B cells before they can produce antibodies
• Regulatory T cells moderate the immune response and establish and control sensitivity of immune response
• Memory T cells respond to antigens they have already encountered.

Lymphocyte Lymphopoiesis

• When stimulated B cells become plasma cells that produce and secrete antibodies and are responsible for antibody-mediated immunity which circulate in body fluids to attack targets throughout the body.
• NK, natural killer cells, attack foreign cells, virus-infected body cells, and cancer cells.
• NK cells continuously monitor peripheral tissues and perform immune surveillance.
• Lymphopoiesis is lymphocyte production which involves red bone marrow, thymus, and peripheral lymphoid tissues; bone marrow maintains the normal lymphocyte population and hematopoietic stem cells produce lymphoid stem cells.

Lymphoid Stem Cells

• Lymphoid stem cells produce all lymphocyte types from two groups.
• One group migrates to the thymus and will develop into T cells; the second group stays in the red bone marrow and divides to produce B cells, which mature and move into lymph nodes and spleen and NK cells, which patrol peripheral tissues.
• Development of stem cells in the thymus are isolated from general circulation by a the blood thymus barrier.
• Stem cells then divide in response to thymic hormones to produce T cells after they undergo a selection process that ensures they won't react to normal body cells.

Lymphocyte Survival

• Up to 98 percent of T cells are deselected and die, but the mature T cells re-enter bloodstream and then travel to peripheral lymphoid tissues and organs; the T cells and B cells then migrate from their sites of origin.
• T cells and B cells retain the ability to divide and create daughter cells of the same type, and the ability to increase in number is crucial to a successful immune response.

Lymphoid Tissues

• Lymphoid tissues are connective tissues dominated by lymphocytes.
• Lymphoid nodules are densely packed lymphocytes in an area of areolar tissue in which nodules may cluster together and form larger masses lacking a fibrous capsule.
• Aggregated lymphoid nodules are clusters of lymphoid nodules deep to the lining of the intestines and each nodule often has a central zone containing dividing lymphocytes.
• Mucosa-associated lymphoid tissue (MALT) protects epithelia of digestive, respiratory, urinary, and reproductive tracts from pathogens and toxins.

Tonsils

• Tonsils are large lymphoid nodules in the walls of the pharynx.
• the Pharyngeal tonsil (or the adenoid) is located on posterior superior wall of the nasopharynx
• the Palatine tonsils (left and right) are located at posterior, inferior margin of the oral cavity along the
• Lingual tonsils are a pair of tonsils located deep to the epithelium covering the base of the tongue
• Tonsillitis is the inflammation of tonsils.
• Lymph nodes are small lymphoid organs surrounded by fibrous connnective tissue capsule and are shape resembles a kidney bean, with a diameter range from 1-25 mm.
• Large lymph nodes (lymph glands) located in neck, groin, axillae are filters, removing about 99% of pathogens from lymph before fluid returns from bloodstream
• Trabeculae are fibrous partitions extending inward from capsule.

Path of Through a Lymph Node

• Afferent lymphatics bring lymph into the lymph node on the opposite side from the hilum, which is the indentation.
• Lymph passses through the subcapsular space, a network of fibers and dendritic cells involved in the immune response.
• The fluid then goes into the outer cortex, containing B cells within germinal centers and then flows to the paracortex that contains T cells.
• The fluid continues into the medullary sinus at the cord which contains B cells and plasma cells.
• The the lymph exits through the efferent lymphatics at the hilum and goes into venous circulation.
• MALT defends exposed epithelia in multiple tracts exposed to the exterior environment but infection and/or inflammation of MALT components, examples are tonsillitis and appendicitis, can cause varieties of clinical disorders.

Thymus

• The thymus is a lymphoid organ which produces functional T cells, in which size and secretory abilities decline with age.
• The Thymus size and function is largest (40 g) before puberty after which, diminishes in size and becomes increasingly fibrous (involution).
• By age 50, size can be <12 g which correlates with increased susceptibility to disease.
• The Thymus is located in the mediastinum and posterior to the sternum.
• It is covered in a capsule which divides it into left and right divided lobes with fibrous partitions (septa) that divides lobes into lobules with each lobule about 2 mm in diameter.

Thymus Histology

• Each lobe consists of a dark outer cortex with dividing lymphocytes surrounded by epithelial reticular cells (ERCs) that regulates T cell development and function.
• Blood vessels in the cortex are also surrounded by epithelial cells in order to maintain the blood thymus barrier; there is a lighter central medulla.
• Thymic epithelial cells form thymic corpuscles, distinct clusters of cells in concentric layers.
• The autoimmune disease myasthenia gravis is associated with an enlarged, hyperactive thymus and is characterized by skeletal muscle weakness with Acetylcholine (ACh) receptors at nueromuscular junctions that are destroyed, altered, or blocked by self antibodies.

Spleen

• The spleen is the largest lymphoid organ and responds to blood stream antigens, as well as contains the largest mass of lymphoid tissue in the body which performs the same function on blood as lymph nodes filter lymph.
• Spleen removes abnormal red blood cells and other blood components via phagocytosis, stores iron that is recycled from red blood cells, and initiates immune responses by B cells and T cells in response to antigens in circulating blood.
• lies along curvature of stomach's lateral border on left, Attached by short gastrosplenic ligament.
• Diaphragmatic surface smooth and convex, fits diaphragm shape with surface about (12 cm), weighs around (160 g), deep red color.
• Fairly easily ruptured by impact and if Spleen tissue damage, has to be surgically removed (splenectomy) , because tissue to delicate to repair.

Internal Spleen Anatomy

• Capsules are the Outer of collagen and elastic fibers then Trabeculae fibrous radiate capsule outward from hilium.
• There is the Pulp within the capsule of cellular component and there two types:
  • Red Pulp consist of amounts of RBCs, as well as containing Macrophages.
  • White Pulp Resembles lymphoid nodules full of Lymphocytes.

Blood Flow in Spleen

• The trabecular arteries are branches of the splenic artery and their finer branches, called central are surrounded by areas of white pulp; the Capillaries discharge blood into the reticular tissue of the red pulp, which contains macrophages, as well as flow into sinusoids whose walls contain macrophages.
• Finally blood collects into small veins that merge to form trabecular.
• A ruptured spleen is a medical emergency which tears easily and the damaged tissue is difficult or impossible to repair surgically.
• Splenectomy is the removal of the spleen and without it, a person is at increased risk for bacterial infection.

Innate Immunity

• Innate immunity is nonspecific to give an overview of it's components: The components of innate immunity give an overview of nonsepcific immunity.
• Physical barriers play a role in innate immunity.
• Phagocytes play a role in innate immunity.
• Immune surveillance of NK cells.
• Various types of interferons, compliment activation.
• Inflammation as well as fever innate defense mechanisms.
• Innate Immunity does not distinguish a type of threat from another; the response is the same, and is also present at birth which gives Non-sepcific resistance and limits microbes.

Components of Immunity

• Two Mechanisms have a role; physical barriers (skin, mucous membranes), phagocytes engulfing pathogens, cells to destroying by NK cells, Interferons with chemical as viral infections, and complement with circulating proteins to assisting Antibodies, while the bodies inflammation on tissue and increase in temperature with body, then it is immunity fever.
• Integumentary is the body's is the key point of skin for physical barriers and secretion chemicals.
• Hair with protection hazardous against material and insects's are Epithelium and stratified Squamous with keratinized cells that connecting desomosomes.
• Epithelial lining can be cells provide like digestive, respiratory, urinary, and reproductive systems but will have secretion of mucus, stomach acid destroy such as MALT provide non specific.

White Blood Cells

• Neutrophills, Eosinophils, as well as Monocytes change to Macrophage and Free macrophage and help with chemotaxis but with emigration process with Adheasion then to destroy pathogens