The Lymphatic System and Immunity: ANP Study Notes

The Lymphatic System and Immunity

Overview of the Lymphatic System

The lymphatic system is a network of cells, tissues, and organs that protects the body from environmental hazards and internal threats. Its primary cells are lymphocytes, and its vessels are called lymphatic vessels or lymphatics. Lymphoid tissues and organs are distributed throughout the body.

• Lymphocytes: Main cells of the lymphatic system; respond to pathogens, abnormal body cells, and foreign proteins.

• Lymph: Fluid similar to interstitial fluid, surrounds lymphocytes.

• Lymphatic vessels: Transport lymph from peripheral tissues to the venous system.

Fluid Flow and Lymphatic Capillaries

Lymphatic vessels begin as lymphatic capillaries, which are found in most tissues and organs and are closely associated with blood capillaries. Interstitial fluid enters these capillaries and becomes lymph.

• Lymphatic capillaries: Lined by endothelial cells with incomplete or missing basement membranes.

• Differences from blood capillaries:

  1. Originate as pockets, not continuous tubes

  2. Larger diameters

  3. Thinner walls

  4. Appear flattened or irregular in sectional view

• One-way flow: Overlapping endothelial cells act as valves, allowing entry of fluids and solutes but preventing backflow.

• Valves: Present in larger lymphatic vessels, prevent backflow due to low pressure.

• Locations without lymphatic capillaries: Areas without blood supply (e.g., cornea), red bone marrow, central nervous system.

Extracellular Fluid Circulation and Homeostasis

Blood remains in cardiovascular vessels, while water and solutes move into interstitial fluid. Lymph forms as interstitial fluid drains into lymphatic vessels, ensuring fluid homeostasis.

• Eliminates local differences in nutrients, wastes, and toxins

• Maintains blood volume

• Alerts immune system to infections

Lymphatic Drainage and Ducts

Lymphatic vessels converge to form larger trunks, which empty into collecting vessels and major ducts.

Duct	Drains	Empties Into
Thoracic duct	Body inferior to diaphragm, left side superior to diaphragm	Left subclavian vein
Right lymphatic duct	Right side superior to diaphragm	Right subclavian vein

• Cisterna chyli: Expanded chamber at base of thoracic duct; receives lymph from lower body via lumbar and intestinal trunks.

Lymphedema

Lymphedema is caused by blocked lymphatic drainage, leading to accumulation of interstitial fluid, swelling, and increased risk of infection due to overwhelmed local immune defenses.

Lymphocytes and Immune Cells

Classes of Lymphocytes

Lymphocytes account for 20–40% of circulating leukocytes, but most reside in lymphoid tissues. There are three main classes:

Type	Percentage	Main Function
T cells	80%	Cell-mediated immunity; attack infected or abnormal cells
B cells	10–15%	Antibody-mediated immunity; produce antibodies
NK cells	5–10%	Immunological surveillance; attack foreign, infected, or cancerous cells

Antigens

Antigens are substances (usually proteins, but also lipids, polysaccharides, or nucleic acids) that stimulate an immune response. All lymphocyte classes are sensitive to antigens.

Lymphopoiesis

Lymphocyte production occurs in red bone marrow, thymus, and peripheral lymphoid tissues.

• Red bone marrow: Hemocytoblasts produce lymphoid stem cells.

• Some stem cells migrate to thymus (become T cells); others remain to become B cells and NK cells.

• Thymus: Thymic hormones stimulate T cell production and differentiation.

• Lymphocyte reproduction: B and T cells retain ability to divide, ensuring immune response capability.

Lymphoid Tissues and Organs

Lymphoid Tissue and Nodules

Lymphoid tissue is connective tissue dominated by lymphocytes. Lymphoid nodules are clusters found in tracts exposed to the external environment, collectively called mucosa-associated lymphoid tissue (MALT).

The Tonsils

Tonsils are large lymphoid nodules in the pharynx, with three types:

• Pharyngeal tonsil (adenoid): Posterior, superior wall of nasopharynx

• Palatine tonsils: Posterior, inferior margin of oral cavity

• Lingual tonsils: Base of tongue

Tonsillitis: Inflammation due to infection.

Lymph Nodes

Lymph nodes are small, kidney bean-shaped organs that filter lymph, removing 99% of antigens and initiating immune responses as needed.

• Afferent lymphatics: Bring lymph into node

• Efferent lymphatics: Carry lymph out toward veins

• Contain B cells and plasma cells

The Thymus

The thymus produces hormones (thymosins) essential for T cell development. Its size and function decline with age, increasing disease susceptibility.

The Spleen

The spleen is the largest lymphoid organ, performing for blood what lymph nodes do for lymph.

• Removes abnormal red blood cells by phagocytosis

• Stores iron recycled from RBCs

• Initiates immune responses by B and T cells

Immunity: Nonspecific and Specific Defenses

Nonspecific (Innate) Immunity

Nonspecific defenses do not distinguish between pathogens and are present at birth. They provide general resistance to infection.

• Physical barriers (skin, mucous membranes)

• Phagocytes (neutrophils, eosinophils, macrophages)

• Immunological surveillance (NK cells)

• Interferons

• Complement system

• Inflammatory response

• Fever

Physical and Epithelial Barriers

• Integumentary system: Skin, glands, and hair provide physical and chemical protection.

• Epithelial barriers: Line tracts; secretions contain enzymes, antibodies, or acid (e.g., stomach acid, mucus, urine, glandular secretions).

Phagocytes

Phagocytes are the first line of cellular defense, removing debris and pathogens before lymphocytes are activated.

• Neutrophils: Abundant, fast-acting, phagocytize bacteria

• Eosinophils: Target antibody-coated pathogens

• Monocyte–macrophage system: Fixed and free macrophages

Diapedesis: Phagocytes leave capillaries by squeezing between endothelial cells. Chemotaxis: Movement toward or away from chemical signals.

Inflammation and Fever

• Inflammation: Localized response to injury; causes swelling, redness, heat, and pain. Triggered by chemical changes in interstitial fluid.

• Fever: Body temperature rises above 37.2ºC due to pyrogens; inhibits pathogens and accelerates defenses and repair.

Summary Table: Nonspecific Defenses

Defense	Function
Physical barriers	Block entry of pathogens
Phagocytes	Engulf pathogens and debris
NK cells	Destroy abnormal cells
Interferons	Trigger antiviral protein production
Complement system	Assist antibodies in destroying pathogens
Inflammation	Limit spread of injury/infection
Fever	Accelerate metabolism and defenses

Specific (Adaptive) Immunity

Specific immunity targets particular pathogens and is coordinated by T cells (cell-mediated immunity) and B cells (antibody-mediated immunity).

• Active immunity: Body produces antibodies after exposure to antigen (naturally or artificially acquired).

• Passive immunity: Antibodies received from another source (naturally or artificially acquired).

Properties of Specific Immunity

• Specificity: Each lymphocyte targets a specific antigen.

• Versatility: Millions of lymphocyte populations, each sensitive to different antigens.

• Immunological memory: Memory cells enable faster, stronger responses upon re-exposure.

• Tolerance: Immune system ignores "self" antigens.

Antigen Presentation and Immune Activation

• Antigen-presenting cells (APCs): Monocytes, macrophages, dendritic cells; engulf pathogens, display antigenic fragments on their surface.

• Defenses against bacteria: Initiated by macrophages and antigen presentation.

• Defenses against viruses: Involve direct contact with infected cells, antigen presentation, and interferon release.

Memory Cells

Memory cells do not participate in the initial response but enable rapid, effective responses to subsequent exposures by differentiating into cytotoxic T cells, helper T cells, or plasma cells.

Antibodies and Their Actions

Antibody Structure

An antibody molecule consists of two pairs of polypeptide chains (one pair of heavy chains, one pair of light chains), each with constant and variable segments. The variable segments form antigen-binding sites.

• Antigenic determinant site: Specific region of antigen recognized by antibody.

• Antigen-antibody complex: Formed when antibody binds to antigenic determinant site.

Types of Antigens

• Complete antigen: Has at least two antigenic determinant sites.

• Partial antigen (hapten): Cannot activate B cells alone; may cause allergic reactions when attached to carrier molecules.

Classes of Immunoglobulins (Antibodies)

Class	Function
IgG	Main antibody in secondary response; crosses placenta
IgE	Involved in allergic responses; binds to mast cells and basophils
IgD	Found on B cell surface; role in B cell activation
IgM	First antibody produced in primary response; causes agglutination
IgA	Found in secretions (saliva, mucus, tears); protects mucosal surfaces

Primary and Secondary Immune Responses

• Primary response: Initial exposure; antibody titer peaks 1–2 weeks after exposure, then declines.

• Secondary response: Faster, stronger, and longer-lasting due to memory B cells; antibody titer rises rapidly.

Antibody Mechanisms

Antibodies eliminate antigens through seven mechanisms:

1. Neutralization: Block pathogen binding sites.

2. Prevention of pathogen adhesion: Form barriers on body surfaces.

3. Activation of complement: Trigger complement system to destroy pathogens.

4. Opsonization: Coat pathogens to enhance phagocytosis.

5. Attraction of phagocytes: Recruit neutrophils, eosinophils, and macrophages.

6. Stimulation of inflammation: Induce release of histamine and heparin.

7. Agglutination: Clump antigens together (e.g., transfusion reactions).

Immune Disorders

Allergies and Hypersensitivity

• Allergy: Inappropriate or excessive immune response to antigens (allergens).

• Immediate hypersensitivity: Rapid, severe response (e.g., allergic rhinitis).

• Anaphylaxis: Systemic allergic reaction; can cause anaphylactic shock due to widespread vasodilation and drop in blood pressure.

Autoimmune Disorders

Immune system attacks self-tissues due to malfunction in self-recognition, producing autoantibodies.

• Examples: Thyroiditis, rheumatoid arthritis, type I diabetes

• Mechanism: Similarities between pathogen and self-proteins may trigger cross-reactivity (e.g., multiple sclerosis).

Graft Rejection

After organ transplant, recipient's T cells may attack donor tissue. Immunosuppression (e.g., cyclosporin A) reduces immune response to improve transplant success.

Immunodeficiency Diseases

• Causes: Developmental problems, viral infections (e.g., HIV/AIDS), or immunosuppressive treatments.

• AIDS: Caused by HIV, which infects and destroys helper T cells, impairing both cell-mediated and antibody-mediated immunity.

Age-Related Immune Decline

• Thymus shrinks, thymic hormone production decreases

• T and B cells less responsive; antibody production slows

• Increased susceptibility to infections and cancer

• Vaccinations recommended for elderly

Key Definitions and Concepts

• Lymph: Fluid in lymphatic vessels, derived from interstitial fluid

• Antigen: Substance that stimulates an immune response

• Opsonization: Coating of pathogens by antibodies/complement to enhance phagocytosis

• Chemotaxis: Movement of cells toward chemical signals

• Immunosuppression: Reduction of immune response, often to prevent graft rejection

• Autoimmune disorder: Immune system attacks self-tissues

Sample Equations and Formulas

• Antibody Titer: Measurement of antibody concentration in blood $\text{Antibody Titer} = \frac{\text{Amount of Antibody}}{\text{Volume of Serum}}$

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