The Lymphatic System and Immunity

• The lymphatic system defends the body against pathogens and internal threats like cancer cells
• Immunity is the ability to resist infection and disease
• The cells and tissues producing immunity are part of the immune system

Components of the Lymphatic System

• Lymphatic vessels, tissues, and organs maintain fluid volume and contribute to body defenses
• The lymphatic system includes lymph, lymphatic vessels, lymphoid tissues/organs (primary/secondary), and lymphoid cells
• Lymphocytes form and mature in primary lymphoid tissues/organs
• Lymphocytes activate and clone in secondary lymphoid tissues/organs

Functions

• Lymphatic system produces, maintains, and distributes lymphocytes and other lymphoid cells for defense
• It picks up excess fluid in peripheral tissue and returns it to venous circulation to maintain blood volume
• It eliminates local interstitial fluid variations by distributing nutrients and wastes to general circulation

Flow of Lymph

• Lymphatic capillaries (smallest) > lymphatic vessels (middle) > lymph nodes > lymphatic trunks > thoracic duct & right lymphatic duct (largest) > right and left subclavian veins

Lymphatic vs Regular Capillaries

• Lymphatic capillaries have overlapping endothelial cells acting as one-way valves, allowing entry but preventing return
• Lymphatic capillaries are closed at one end
• They have thinner walls and larger luminal diameters than regular capillaries

Lacteals

• Lacteals are special lymphatic capillaries in the small intestine
• They transport lipids from digestion causing lymph to appear milky-white

Lymphatic Vessels

• Lymphatic vessels have internal one-way valves
• Superficial lymphatics are located in the skin, mucous membranes, and serous membranes
• Deep lymphatics are larger vessels alongside arteries and veins
• Lymphatic trunks form from lymphatic vessels, emptying into two major collecting vessels: the thoracic duct and the right lymphatic duct
• The thoracic duct empties into the left subclavian vein
• The right lymphatic duct empties into the right subclavian vein

Lymphedema

• Lymphedema is caused by a blockage of lymph drainage in a limb
• Lymphedema results in severe swelling
• Lymphedema interferes with immune system function

Lymphoid Cells

• Lymphoid cells are immune system cells and supportive cells in lymphoid tissues
• Lymphocytes make up 20-40% of circulating leukocytes
• Most lymphocytes are stored, not circulating
• Thymus-dependent T cells
• Bone marrow-derived B cells
• Natural killer cells (NK cells)

Lymphoid Tissue

• Lymphoid tissues are connective tissues dominated by lymphocytes
• Lymphoid nodules (lymphatic nodules) consist of areolar tissue with densely packed lymphocytes
• Lymphoid nodules have a germinal center that contains dividing lymphocytes
• Lymphoid nodules are distributed in lymph nodes, the spleen, along digestive/urinary/reproductive tracts and in the respiratory tract (tonsils)

Tonsils

• Five tonsils are located in the wall of the pharynx: the pharyngeal tonsil (adenoid), left/right palatine tonsils, and two lingual tonsils
• Tonsillitis is inflammation of tonsils, especially palatine tonsils

Mucosa-Associated Lymphoid Tissue (MALT)

• MALT consists of lymphoid tissues associated with the digestive system
• Aggregated Lymphoid Nodules are clustered deep to intestinal epithelial lining

Appendix

• The appendix contains a mass of fused lymphoid nodules

Lymphoid Organs

• Lymphoid organs are separated from surrounding tissues by a fibrous connective tissue capsule
• Lymphoid Organs include lymph nodes, thymus, and spleen

Lymph Nodes

• Lymph nodes are encapsulated masses of lymphoid tissue with connective tissues
• Trabeculae are bundles of collagen fibers extending from capsule into the interior of lymph node
• The hilum is a shallow indentation where blood vessels and nerves reach lymph node
• Afferent lymphatic vessels carry lymph from peripheral tissues to lymph node
• Efferent lymphatic vessels leave lymph node at the hilum and carry lymph to venous circulation

Lymph Node Function

• Lymph nodes purify lymph before returning it to venous circulation, removing 99% of antigens
• Antigens released due to infection enter lymph and stimulate macrophages/lymphocytes in lymph nodes
• Dendritic cells carry antigens to lymph nodes
• Lymph nodes monitor peripheral infections and respond before infections reach vital organs
• Lymph nodes of gut, trachea, lungs, and thoracic duct protect against pathogens in digestive/respiratory systems
• The largest lymph nodes are in the groin, axillae, and base of neck, swelling in response to infection
• Lymphadenopathy is chronic/excessive enlargement of lymph nodes that may indicate infections or cancer

Thymus

• The thymus is located in the mediastinum
• It atrophies after puberty, diminishing the effectiveness of the immune system
• The thymus is divided into two thymic lobes
• Septa divides lobes into smaller lobules

Epithelial Reticular Cells

• Epithelial reticular cells surround lymphocytes in the cortex to form layered structures in the medulla known as thymic (Hassall’s) corpuscles
• They maintain the blood-thymus barrier in the cortex
• They produce thymic hormones (= Thymosin), an extract containing several hormones promoting T cell development and maturation

Spleen

• The spleen is the largest mass of lymphoid tissue in the body
• The red pulp consists of red blood cells
• The white pulp is similar to lymphoid nodules

Functions of the Spleen

• Spleen removes abnormal blood cells and other blood components by phagocytosis
• Spleen stores iron recycled from red blood cells
• Spleen initiates immune responses by B/T cells in response to antigens in circulating blood
• Splenectomy is the removal of a severely ruptured spleen