Lymphatic System: Function and Structures

Lymphatic System Overview

• The lymphatic system functions as a drainage system, transporting proteins and fluids that have leaked from blood capillaries
• Fats are absorbed from the intestinal tract and transported to the bloodstream via the lymphatic system
• The lymphatic system defends the body against foreign organisms through lymphocyte storage

Structures of the Lymphatic System

• Interstitial fluid is a clear, colorless fluid found in tissues between cells
• Interstitial fluid becomes lymph when it enters lymph capillaries
• Lymph capillaries are thin-walled tubes that allow interstitial fluid to enter lymph vessels
• Lymph vessels are similar to veins and contain valves to prevent backflow
• Lymph vessels carry lymph throughout the body
• Lymph flows in one direction, unlike blood
• Lymph always travels toward the thoracic cavity, draining into the bloodstream
• Lymph vessels get larger as they move toward the thoracic cavity until they drain into venous blood vessels near the heart
• Lymph travels past the lungs and then throughout the body via the bloodstream
• Lacteals are specialized lymph vessels in the small intestine that transport fats and fat-soluble vitamins
• Lacteals have small nodes in villi in the intestines

Lymph Nodes, Tonsils, Spleen, and Thymus

• Lymph nodes (lymphaden/o) are small, stationary, bean-shaped structures that filter lymph and store lymphocytes
• Lymph nodes are described by their location in the body (e.g., pregcapular and maxillary)
• Tonsils (tonsill/o) are masses of lymphatic tissue that protect the nose and cranial throat
• Tonsils are named or described according to their location
• The spleen (splen/o) is a ductless vascular gland located in the upper left abdomen near the stomach
• The spleen breaks down damaged or old RBCs
• The spleen filters microorganisms
• The spleen activates lymphocytes
• The spleen stores blood
• The thymus (thym/o) matures T lymphocyte cells
• The thymus is located in the cranioventral portion of the thoracic cavity

Immune System Overview

• The immune system is not contained in one set of organs
• Structures from different body systems, including integumentary, lymphatic, respiratory, and GI, participate in immune responses
• Immun/o means protects
• Immunity implies one is "exempt" or protected from foreign substances

Innate Immunity: Non-Specific Defenses

• Innate immunity is the first line of defense and is non-specific
• Mechanical and physical barriers are part of the first line of defense
• These barriers include skin, mucous membranes and mucus, lacrimal apparatus (tears), saliva, cilia of the respiratory tract, epiglottis, urine flow, vaginal secretions, and GI tract via peristalsis, defecation, and vomiting
• Chemical barriers are also part of the first line of defense
• Sebum has antibacterial properties due to its low pH
• Perspiration contains lysozyme, which breaks down the cell walls of many bacteria
• Gastric fluids contain hydrochloric acid and enzymes
• Vaginal secretions have an acidic pH
• Leukocytes are the second line of defense
• Phagocytic leukocytes include granulocytes, monocytes, and macrophages
• Natural killer cells are a specific type of lymphocyte
• Inflammation is a local response
• Fever is a systemic response
• Antimicrobial substances are part of the second line of defense
• The complement system is an antimicrobial substance
• Interferons are antimicrobial substances
• Iron-binding proteins are antimicrobial
• Antimicrobial peptides are components of the second line of defense

Adaptive Immunity: Specific Defenses

• Adaptive immunity is the third line of defense and is specific
• It requires proper function of monocytes and macrophages
• Cell-mediated immune response involves T lymphocytes including T Helper (TH), T Regulatory (TR), T Cytotoxic (TC), T Delayed Hypersensitivity (TD), and T Memory (TM) cells
• Humoral immunity involves B lymphocytes
• Types of cells in humoral immunity are B Memory and Plasma cells ("plasmocytes")
• Plasma cells include IgM, IgG, IgA (immunoglobulin), IgE, and IgD
• Passive immunity is receiving immunity from another source and is short-term
• Natural passive immunity includes colostrum, which contains many antibodies from the mother's milk
• Artificial passive immunity includes anti-serum, which provides only the mother's antibodies
• Active immunity is developing immunity from within the body and is long-term
• Natural active immunity occurs from being exposed to a disease
• Artificial active immunity comes from vaccination

Learn about the lymphatic system's role in draining fluids and transporting fats. Explore the structures involved, including interstitial fluid, lymph capillaries, and vessels. Understand how lymph flows and its importance in immune defense.