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Transcript
Chapter 22 The Lymphatic System and Immunity The Lymphatic System • A system consisting of lymphatic vessels through which a clear fluid (lymph) passes • The major functions of the lymphatic system include: • Draining interstitial fluid • Transporting dietary lipids absorbed by the gastrointestinal tract to the blood • Facilitating the immune responses The Lymphatic System • Components of the lymphatic system include: • Lymphatic capillaries • Lymphatic vessels • Lymph nodes • Lymphatic trunks • Lymphatic ducts • Primary lymphatic organs • Secondary lymphatic organs and tissues Lymphatic Vessels and Fluid • Lymph is a clear to milky fluid in the extracellular fluid compartment. Extracellular fluids include: • Plasma – the liquid component of blood • Interstitial fluid – the clear fluid filtered through capillary walls when it enters the “interstitium” (space between cells, also called the intracellular space) • Lymphatic fluid – the unaltered interstitial fluid that enters the lymphatic vessels. In the GI tract, lymphatic fluids also include absorbed dietary lipids. Lymphatic Vessels and Fluid • The flow of lymph fluid is always from the periphery towards the central vasculature. • It starts as interstitial fluid. • Then enters lymphatic capillaries. • It travels in lymphatic vessels to the regional lymph nodes… Lymphatic Vessels and Fluid • The flow of lymph fluid continued… • Lymph ascends or descends to the thorax, either to the Left or Right Lymphatic Duct. • Lymph fluid’s final destination is the bloodstream, as it enters through the Subclavian veins. Lymphatic Vessels and Fluid • Lymphatic capillaries are slightly larger than blood capillaries and have a unique one-way structure. • The ends of endothelial cells overlap and permit interstitial fluid to flow in, but not out. • Anchoring filaments pull openings wider when interstitial fluid accumulates. • There are specialized lymphatic capillaries called lacteals that take up dietary lipids in the small intestine. • Chyle is the name of this “lymph with lipids”. Lymphatic Vessels and Fluid Lymphatic capillaries showing blind ends and one way flow Lymphatic Vessels and Fluid • Lymphatic capillaries unite to form larger lymphatic vessels which resemble veins in structure but have thinner walls and more valves. • Lymphatic vessels pass through lymph nodes – encapsulated organs with masses of B and T cells. • Function as lymph filters Lymphatic Vessels and Fluid Lymphatic Vessels and Fluid • Lymphatic fluid is moved by pressure in the interstitial space and the milking action of skeletal muscle contractions and respiratory movements. • An obstruction or malfunction of lymph flow leads to edema from fluid accumulation in interstitial spaces. Lymphatic Organs • The lymphatic system is composed of a number of primary and secondary organs and tissues widely distributed throughout the body - all with the purpose of facilitating the immune response. Lymphatic Organs • Primary lymph organs are the bone marrow and thymus. – Sites where stem cells divide and become immunocompetent (capable of mounting an immune response) • Secondary lymphatic organs are sites where most immune responses occur, including the spleen and lymph nodes, and other lymphoid tissues such as the tonsils. Lymphatic Organs • Thymus • The outer cortex is composed of a large number of immature T cells which migrate from their birthplace in red bone marrow . • They proliferate and begin to mature with the help of Dendritic cells (derived from monocytes) and specialized epithelial cells (help educate T cells through positive selection) – only about 25% survive. • The inner medulla is composed of more mature T cells. Lymphatic Organs • The thymus slightly protrudes from the mediastinum into the lower neck. • It is a palpable 70g in infants, atrophies by puberty, and is scarcely distinguishable from surrounding fatty tissue by old age. Lymphatic Organs • There are about 600 lymph nodes scattered along lymphatic vessels (in superficial and deep groups) that serve as filters to trap and destroy foreign objects in lymph fluid. • Important group of regional lymph nodes include: – Submandibular – Cervical – Axillary • Mediastinal • Inguinal Lymphatic Organs • Lymph fluid enters the node through afferent vessels and is directed towards the central medullary sinuses. • Efferent vessels convey lymph, antibodies and activated T cells out of the node at an indentation called the hilum. Lymphatic Organs • The spleen is the body’s largest mass of lymphatic tissue. • The parenchyma of the organ consists of: • White pulp - lymphatic tissue where lymphocytes and macrophages carry out immune function • Red pulp – blood-filled venous sinuses where platelets are stored and old red cells are destroyed Lymphatic Organs The Immune Response • Our immune response includes innate and adaptive responses: Innate Immunity • The innate immune response is present at birth. It is non-specific and non-adaptive. • It includes our first line of external, physical, and chemical barriers provided by the skin and mucous membranes. Innate Immunity • Our nonspecific innate immune response also includes various internal defenses such as antimicrobial substances, natural killer cells, phagocytes, inflammation, and fever. Innate Immunity • Internal defenses: – Phagocytes • Wandering and fixed macrophages – Natural killer (NK) cells – Endogenous antimicrobials – Complement system – Iron-binding proteins – Interferon Innate Immunity • Phagocytosis is a non-specific process wherein neutrophils and macrophages (from monocytes) migrate to an infected area. There are 5 steps: • • • • • Chemotaxis Adherence Ingestion Digestion Killing Innate Immunity Innate Immunity • Fever is an abnormally high body temperature due to resetting of the hypothalamic thermostat. • Non-specific response: • speeds up body reactions • increases the effects of endogenous antimicrobials • sequesters nutrients from microbes Innate Immunity • Inflammation is defensive response of almost all body tissues to damage of any kind (infection, burns, cuts, etc.). • The four characteristic signs and symptoms of inflammation are redness, pain, heat, and swelling. • It is a non-specific attempt to dispose of microbes and foreign materials, dilute toxins, and prepare for healing. Innate Immunity • The inflammatory response has three basic stages: • Vasodilation and increased permeability • Emigration (movement) of phagocytes from the blood into the interstitial space and then to site of damage • Tissue repair Innate Immunity Vasodilation allows more blood to flow to the damaged area which helps remove toxins and debris. • Increased permeability permits entrance of defensive proteins (antibodies and clotting factors) to site of injury • Other inflammatory mediators include histamine, kinins, prostaglandins (PGs), leukotrienes (LTs), and complement. Innate Immunity • Emigration of phagocytes depends on chemotaxis • Neutrophils predominate in early stages but die off quickly. • Monocytes transform into macrophages and become more potent phagocytes than neutrophils. • Pus is a mass of dead phagocytes and damaged tissue. • Pus formation occurs in most inflammatory responses and usually continues until the infection subside.