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

Structure and Function of the Lymphatic System

Introduction to the Immune and Lymphatic Systems

The immune and lymphatic systems work together to provide immunity, protecting the body from cellular injury and pathogens. The immune system consists of cells and proteins in blood and tissues, while the lymphatic system is a group of organs and tissues that also maintains fluid homeostasis.

• Immune System: Includes leukocytes (white blood cells) and immune proteins in plasma.

• Lymphatic System: Composed of lymphatic vessels (blind-ended tubes) and lymphatic tissue/organs (tonsils, lymph nodes, spleen, thymus).

Functions of the Lymphatic System

The lymphatic system has three main functions:

• 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: Dietary fats enter lymphatic vessels (lacteals) in the small intestine and are delivered to the blood.

• Immune Functions: Lymphoid organs filter pathogens from lymph and blood.

Lymphatic Vessels and Lymph Circulation

Lymphatic vessels collect lymph and merge to form lymph trunks, which drain specific body regions. The main trunks include lumbar, jugular, intestinal, bronchomediastinal, and subclavian trunks. The cisterna chyli is a large vessel that drains into the thoracic duct, which empties into the left internal jugular and subclavian veins. The right lymphatic duct drains the upper right side of the body.

Lymphatic vessels are low-pressure circuits with valves to prevent backflow. Lymphatic capillaries are blind-ended, forming a one-way system. Their walls are leaky, allowing fluid and immune cells to enter.

Lymphedema

Lymphedema is swelling caused by accumulation of excess interstitial fluid, often due to removal or blockage of lymphatic vessels. It prevents fluid return to the cardiovascular system, causing tissue enlargement.

Lymphoid Tissues and Organs

Reticular Tissue and Cells

Lymphatic tissue is a type of loose connective tissue called reticular tissue, containing specialized cells and reticular fibers that trap pathogens. Lymphoid organs house leukocytes, including macrophages, B and T lymphocytes, dendritic cells, and reticular cells.

Mucosa-Associated Lymphatic Tissue (MALT)

MALT consists of clusters of lymphoid tissue protecting mucous membranes. Specialized MALT includes:

• Tonsils: Pharyngeal, palatine, and lingual tonsils around oral and nasal cavities.

• Peyer's Patches: Located in the ileum of the small intestine.

• Appendix: Defends against bacteria in the large intestine.

Lymph Nodes

Lymph nodes are bean-shaped clusters along lymphatic vessels, trapping pathogens in reticular "nets" and preventing their spread. They filter lymph and house immune cells.

Spleen

The spleen is the largest lymphoid organ, filtering pathogens from blood and destroying old erythrocytes. It has two regions: red pulp (macrophages) and white pulp (leukocytes and dendritic cells).

Thymus

The thymus generates functional T cells and is most active in children. It consists of lobules with cortex (dense T cells) and medulla (site of T cell destruction).

Overview of the Immune System

Lines of Defense

The immune system has three lines of defense:

• First Line: Surface barriers (skin and mucous membranes).

• Second Line: Innate immunity (cells and proteins).

• Third Line: Adaptive immunity (cells and proteins).

Types of Immunity

• Innate (Nonspecific) Immunity: Responds to all pathogens in the same way, using antimicrobial proteins and cells.

• Adaptive (Specific) Immunity: Responds to unique antigens, involving cell-mediated (T cells) and antibody-mediated (B cells) immunity. Adaptive immunity has memory and is more efficient upon subsequent exposures.

Surface Barriers

Surface barriers block pathogen entry. Skin is resistant due to keratin and acidic sebum. Mucous membranes secrete mucus and acid to trap and destroy pathogens.

Cells and Proteins of Innate and Adaptive Immunity

• Leukocytes: Agranulocytes (B and T lymphocytes, monocytes) and granulocytes (neutrophils, eosinophils, basophils).

• Other Cells: NK cells, dendritic cells.

• Proteins: Antibodies, complement system, cytokines.

Innate Immunity: Internal Defenses

Cells of Innate Immunity

• Phagocytes: Macrophages, neutrophils, eosinophils ingest pathogens.

• Macrophages: First responders, kill pathogens, present antigens.

• Neutrophils: Effective against bacteria, recruited to damaged tissues.

• Dendritic Cells: Present antigens to T and B cells.

• Eosinophils: Respond to parasitic pathogens.

• NK Cells: Recognize and destroy cancerous and infected cells.

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

Antimicrobial Proteins

• Complement System: 20+ plasma proteins activated by classical, lectin, or alternative pathways. Effects include cell lysis, enhanced inflammation, neutralization of viruses, opsonization, and clearance of immune complexes.

Cytokines

• Tumor Necrosis Factor: Attracts and activates phagocytes.

• Interferons: Inhibit viral replication.

• Interleukins: Stimulate immune cell production and activation.

The Inflammatory Response

Inflammation occurs in response to cell damage. It involves release of mediators (histamine, cytokines), vasodilation, increased permeability, pain, and recruitment of leukocytes.

Anti-inflammatory Medications

• NSAIDs: Inhibit cyclooxygenase, reducing prostaglandin production.

• Corticosteroids: Inhibit formation of prostaglandins and leukotrienes.

Fever

Fever is an innate response to injury, triggered by pyrogens acting on the hypothalamus. It raises body temperature to enhance immune efficiency.

Summary Table: First and Second Lines of Defense

Component	Description	Function(s)
Skin	Stratified squamous, keratinized epithelium	Physical barrier, resists trauma, secretes antimicrobial agents
Mucous membranes	Epithelial membranes lining body passages	Trap pathogens, secrete mucus
Phagocytes	Macrophages, neutrophils, eosinophils	Ingest pathogens and debris
NK cells	Nonphagocytic lymphocytes	Destroy cancer and infected cells
Complement	Plasma proteins	Cell lysis, inflammation, opsonization
Cytokines	Signaling proteins	Regulate immune cell activity
Fever	Elevated body temperature	Enhances phagocyte activity

Adaptive Immunity: Cell-Mediated Immunity

T Cell Response

• T Cells: Formed in bone marrow, mature in thymus. Clones respond to specific antigens.

• Antigen: Substance recognized by B or T cells; immunogens generate immune response.

• MHC Molecules: Class I (all nucleated cells, endogenous antigens), Class II (APCs, exogenous antigens).

T Cell Activation

• Clonal Selection: Dendritic cells present antigens to T cells, activating specific clones.

• Effector and Memory T Cells: Activated T cells proliferate and differentiate.

Effects of T Cells

• Helper T Cells (TH): Secrete cytokines, activate macrophages, TC cells, and B cells.

• Cytotoxic T Cells (TC): Kill infected, cancerous, or foreign cells by releasing perforin and enzymes.

Adaptive Immunity: Antibody-Mediated Immunity

B Cell Activation and Antibody Production

• B Cells: Mature in bone marrow, recognize specific antigens.

• Activation: B cell binds antigen, presents it to TH cell, differentiates into plasma and memory B cells.

Antibody Structure and Classes

• Structure: Y-shaped molecule with two heavy and two light chains; constant and variable regions.

• Classes: IgG, IgA, IgM, IgE, IgD (GAMED mnemonic).

Functions of Antibodies

• Agglutination/Precipitation: Clumping of cells or molecules for easier phagocytosis.

• Opsonization: Coating pathogens to enhance phagocytosis.

• Neutralization: Binding toxins or viruses to prevent harm.

• Complement Activation: IgM and IgG activate complement proteins.

• Stimulation of Inflammation: IgE triggers release of mediators from mast cells and basophils.

Immunological Memory

• Primary Response: Slow, initial exposure to antigen.

• Secondary Response: Rapid, efficient response upon re-exposure, mainly IgG.

• Vaccination: Induces memory cells for future protection.

Disorders of the Immune System

Hypersensitivity Disorders

• Type I (Immediate): Allergies; IgE-mediated, rapid response, can cause anaphylactic shock.

Immunodeficiency Disorders

• Primary: Genetic or developmental.

• Secondary: Acquired (e.g., AIDS caused by HIV-1).

Autoimmune Disorders

• Autoimmunity: Immune system attacks self antigens, leading to diseases like multiple sclerosis and type 1 diabetes.

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