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 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 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 Vessels and Fluid

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 birth- place 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

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

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.