Chapter 20: The Lymphatic System and Immunity – Structured Study Notes

The Lymphatic System and Immunity

Introduction to the Immune and Lymphatic Systems

The immune and lymphatic systems work together to protect the body from pathogens and maintain homeostasis. The immune system consists of cells and proteins distributed throughout the blood and tissues, while the lymphatic system is a network of vessels and organs that supports immune function and fluid balance.

• Immune System: Defends against internal and external threats; includes leukocytes (white blood cells) and immune proteins in plasma.

• Lymphatic System: Composed of lymphatic vessels and lymphatic tissues/organs (tonsils, lymph nodes, spleen, thymus).

Functions of the Lymphatic System

The lymphatic system performs three main functions: regulation of interstitial fluid volume, absorption of dietary fats, and immune defense.

• Regulation of Interstitial Fluid Volume: Returns 2–4 liters of fluid lost from plasma daily to circulation, preventing drops in blood volume and pressure.

• Absorption of Dietary Fats: Fats too large for blood capillaries enter lymphatic vessels (lacteals) in the small intestine and are transported to the blood.

• Immune Functions: Lymphoid organs filter pathogens and house leukocytes for immune response.

Lymphatic Vessels and Lymph Circulation

Lymphatic vessels form a one-way system that transports lymph from tissues back to the cardiovascular system. Lymph is collected in lymph-collecting vessels, which merge into lymph trunks and ducts.

• Lymph Trunks: Drain specific regions (lumbar, jugular, intestinal, bronchomediastinal, subclavian).

• Cisterna Chyli: Large vessel receiving lymph from intestinal and lumbar trunks.

• Thoracic Duct: Largest duct, drains lower body and left upper body.

• Right Lymphatic Duct: Drains upper right side of body.

• Valves: Prevent backflow; lymph flow aided by muscle contractions and smooth muscle in vessel walls.

• Lymphatic Capillaries: Blind-ended, leaky vessels allowing entry of fluid and immune cells.

Lymphedema

Lymphedema is severe swelling caused by impaired lymphatic drainage, often due to surgical removal or blockage of lymphatic vessels.

• Edema: Accumulation of excess interstitial fluid.

• Causes: Surgery, parasites, trauma, vascular disease, heart failure.

• Effects: Fluid accumulates, causing enlargement and disfigurement of affected body part.

Lymphoid Tissues and Organs

Lymphoid tissues are composed of reticular tissue, which forms networks to trap pathogens. Lymphoid organs house various leukocytes and play key roles in immune defense.

• Cell Types: Macrophages, B and T lymphocytes, dendritic cells, reticular cells.

• MALT: Mucosa-associated lymphatic tissue protects mucous membranes; includes tonsils, Peyer's patches, appendix.

• Tonsils: Pharyngeal, palatine, and lingual tonsils trap pathogens and can become inflamed (tonsillitis).

• Peyer's Patches & Appendix: Defend against intestinal bacteria.

• Lymph Nodes: Bean-shaped clusters filter lymph and trap pathogens; divided into cortex (B cells), medulla (macrophages, mature B cells).

• Spleen: Largest lymphoid organ; red pulp destroys old erythrocytes, white pulp filters pathogens.

• Thymus: Site of T cell maturation; active in children, atrophies with age.

Overview of the Immune System

The immune system provides three lines of defense against pathogens: surface barriers, innate immunity, and adaptive immunity.

• First Line: Skin and mucous membranes act as physical barriers.

• Second Line: Innate immunity—rapid, nonspecific response by cells and proteins.

• Third Line: Adaptive immunity—specific response by B and T cells; includes immunological memory.

Types of Immunity

Immunity is classified as innate (nonspecific) or adaptive (specific). Adaptive immunity is further divided into cell-mediated and antibody-mediated responses.

• Innate Immunity: Responds to all pathogens similarly; includes phagocytes, NK cells, complement proteins.

• Adaptive Immunity: Responds to specific antigens; includes cell-mediated (T cells) and antibody-mediated (B cells and antibodies).

• Immunological Memory: Adaptive immunity remembers antigens for faster response upon re-exposure.

Surface Barriers

Surface barriers are the body's first defense against pathogens, consisting of skin and mucous membranes.

• Skin: Keratinized epithelium, resistant to mechanical stress; sebaceous glands secrete acidic sebum.

• Mucous Membranes: Line body passages; secrete mucus to trap pathogens; stomach mucosa secretes acid to kill ingested pathogens.

Cells and Proteins of Innate and Adaptive Immune Systems

Leukocytes are the main cells of the immune system, divided into granulocytes and agranulocytes. Other proteins include antibodies, complement, and cytokines.

• Phagocytes: Macrophages, neutrophils, eosinophils ingest pathogens.

• NK Cells: Destroy cancerous and infected cells.

• Dendritic Cells: Activate T cells.

• Antibodies: Produced by B cells; function in adaptive immunity.

• Complement System: Plasma proteins that enhance immune response.

• Cytokines: Regulate immune cell activity.

How the Lymphatic and Immune Systems Work Together

Lymphoid organs provide residence and activation sites for immune cells, trap pathogens, and are essential for adaptive immunity.

Innate Immunity: Internal Defenses

Innate immunity includes rapid responses by antimicrobial molecules and immune cells. Phagocytic cells ingest pathogens, while nonphagocytic cells mediate inflammation and cytotoxicity.

• Macrophages: First responders; phagocytize pathogens and debris; secrete chemicals to kill pathogens.

• Neutrophils: Most numerous; effective against bacteria; recruited to damaged tissues.

• Dendritic Cells: Present antigens to T cells.

• Eosinophils: Respond to parasitic pathogens.

• NK Cells: Recognize and destroy abnormal cells.

• Basophils/Mast Cells: Mediate inflammation, especially in allergies.

Antimicrobial Proteins

Complement proteins and cytokines are key plasma proteins in innate immunity.

• Complement: Over 20 plasma proteins; critical in both innate and adaptive immunity.

• Tumor Necrosis Factor (TNF): Attracts and activates phagocytes.

• Interferons: Inhibit viral replication.

• Interleukins: Stimulate production of immune cells and trigger inflammation.

The Inflammatory Response

Inflammation is an innate response to tissue damage, involving release of mediators and recruitment of phagocytes.

• Cardinal Signs: Redness, heat, swelling, pain.

• Vasodilation: Increases blood flow to injured area.

• Increased Capillary Permeability: Allows proteins and fluid to enter tissues.

• Pain: Protective function; signals tissue damage.

• Phagocyte Response: Macrophages and neutrophils clear pathogens and debris; monocytes become macrophages.

• Pus: Accumulation of dead cells and fluid.

Fever

Fever is an innate response to infection, regulated by pyrogens acting on the hypothalamus. It is a critical sign of inflammation and helps inhibit pathogen growth.

• Normal Range: 36–38°C (97–99°F).

• Mechanism: Pyrogens reset hypothalamic thermostat; body responds by generating heat.

• Resolution: Hypothalamus returns to normal; body cools via sweating and vasodilation.

Summary Tables: First and Second Lines of Defense

Tables summarize the components and functions of surface barriers and internal defenses.

Component	Description	Function(s)
Skin	Stratified squamous, keratinized epithelium	Physical barrier, resists mechanical trauma
Mucous membranes	Epithelial membranes lining body passages	Trap pathogens, protect underlying cells
Secretions	Sebum, mucus, acid	Deters pathogen growth
Normal flora	Non-pathogenic bacteria	Compete with pathogens

Component	Description	Function(s)
Phagocytes	Macrophages, neutrophils, eosinophils	Ingest pathogens and debris
NK cells	Nonphagocytic lymphocytes	Destroy abnormal cells
Antimicrobial proteins	Complement, cytokines	Enhance immune response
Fever	Elevated body temperature	Inhibits pathogen growth

Adaptive Immunity: Cell-Mediated Immunity

Cell-mediated immunity is carried out by T cells, which mature in the thymus and respond to infected, cancerous, or foreign cells.

• Helper T (TH) Cells: Activate and enhance immune responses via cytokine secretion.

• Cytotoxic T (TC) Cells: Destroy infected or abnormal cells via apoptosis.

• Memory T Cells: Provide rapid response upon re-exposure to antigen.

Adaptive Immunity: Antibody-Mediated Immunity

B cells and antibodies mediate humoral immunity. B cells mature in bone marrow and differentiate into plasma cells and memory B cells upon activation.

• B Cell Activation: Antigen binding triggers clonal selection and differentiation.

• Plasma Cells: Secrete antibodies.

• Memory B Cells: Provide rapid response upon re-exposure.

Antibodies and Their Effects

Antibodies are Y-shaped proteins with variable and constant regions. There are five classes, each with distinct functions.

• IgG: Most prevalent; crosses placenta.

• IgA: Found in secretions; dimer.

• IgM: First antibody produced; pentamer.

• IgE: Involved in allergies and parasitic infections.

• IgD: Acts as B cell receptor.

Immunological Memory

Immunological memory allows for faster and more effective responses upon re-exposure to antigens. Primary and secondary immune responses differ in speed and antibody levels.

• Primary Response: Slow, dominated by IgM.

• Secondary Response: Rapid, dominated by IgG; higher affinity antibodies.

• Vaccination: Induces memory cells for rapid secondary response.

• Active Immunity: Body produces antibodies; long-lasting.

• Passive Immunity: Antibodies received from another source; temporary.

Disorders of the Immune System

Immune system disorders include hypersensitivity (overreaction), immunodeficiency (failure of components), and autoimmune disorders (attack on self antigens).

• Hypersensitivity: Allergies, tissue damage.

• Immunodeficiency: Increased susceptibility to infection.

• Autoimmune Disorders: Immune system attacks body's own tissues.