The Lymphatic System and Immunity: Structure, Function, and Disorders

Chapter 20: The Lymphatic System

Module 20.1 Structure and Function

The lymphatic system is essential for fluid regulation, fat absorption, and immune defense. It consists of vessels, tissues, and organs that work together to maintain homeostasis and protect the body from pathogens.

• Regulation of Fluid Volume: Not all fluid is reabsorbed by blood capillaries; excess fluid enters lymphatic vessels as lymph.

• Absorption of Dietary Fats: Specialized lymphatic capillaries called lacteals absorb fats in the small intestine and transport them to the bloodstream.

• Immune Functions: Lymphatic tissues filter pathogens and prevent infection.

Vessels and Circulation

• Lymphatic Capillaries: Weblike network surrounding blood capillaries; originate as closed-ended pockets, allowing one-way flow of lymph.

• Endothelial Cell "Flaps": Permit entry of fluid and larger solutes, including pathogens.

• Lacteals: Specialized capillaries in the small intestine for fat absorption.

• Lymph-Collecting Vessels: Larger, low-pressure vessels with valves to prevent backflow; located deep within muscle tissues.

• Lymph Trunks: Nine major trunks collect lymph from specific body regions.

• Cisterna Chyli: Collecting chamber for lymph from the lower body.

• Ducts: Two main ducts (thoracic and right lymphatic) drain lymph into the subclavian veins.

• Lymphedema: Accumulation of excess fluid due to lymphatic obstruction or removal, often seen after surgery or in bedridden patients.

Tissues and Organs

• Lymphoid Cells: Include lymphocytes (B and T cells), phagocytes (macrophages, dendritic cells), and reticular cells for structural support.

• Mucosa Associated Lymphatic Tissue (MALT): Clusters of B and T cells in mucous membranes (eyes, GI, oral, nasal, respiratory, urinary systems).

• Specialized MALT: Tonsils (oral cavity), Peyer's patches (small intestine), and appendix (large intestine).

• Lymph Nodes: Small, bean-shaped structures that filter lymph and stimulate immune responses. Located in axillary and cervical regions.

Anatomy of Lymph Node: Cortex (phagocytes, inactive B cells), deeper T cells, medulla (active B cells).

• Flow: Fluid enters via afferent vessels, passes through cortex and medulla, exits via efferent vessel.

• Function: Filters pathogens, prevents their entry into the bloodstream.

• Spleen: Filters blood; red pulp (macrophages, old RBCs), white pulp (leukocytes, pathogens).

• Thymus: Produces hormones for T cell maturation; located in mediastinum.

Module 20.2 Overview of the Immune System

The immune system defends against cellular injury and pathogens through a series of barriers and responses.

• Leukocytes and Proteins: Main defenders against trauma and pathogens (bacteria, viruses, fungi, parasites, toxins, venoms, cancer cells).

• Lines of Defense:

  1. First Line: Skin and mucous membranes (physical barriers).

  2. Second Line: Non-specific cells and proteins (phagocytes, guards).

  3. Third Line: Specific cellular responses (adaptive immunity).

Types of Immunity

• Innate (Nonspecific) Immunity: Rapid, general response to all pathogens; includes surface barriers, cells, and antimicrobial proteins.

• Adaptive (Specific) Immunity: Slow, targeted response to specific antigens; involves immunological memory.

• Antigens: Cell markers identifying self vs. non-self; trigger immune responses.

• Two Arms of Adaptive Immunity: Cell-mediated (T cells) and antibody-mediated (B cells).

First Line of Defense – Surface Barriers

• Skin and Mucous Membranes: Block pathogen entry.

• Protective Secretions: Sebum (acidic, antimicrobial), mucus (traps debris), gastric acid (kills pathogens), defensins (antimicrobial peptides).

• Normal Flora: Beneficial bacteria on skin and mucous membranes, protect against harmful microbes.

Module 20.3 Innate Immunity: Internal Defenses

Innate immunity involves cells and proteins that provide immediate defense against pathogens.

Cells of Innate Immunity

• Phagocytes: Macrophages (APCs), neutrophils, eosinophils, dendritic cells.

• Nonphagocytic Cells: Natural Killer (NK) cells (cytotoxic), basophils (inflammatory mediators), mast cells (allergic response).

Antimicrobial Proteins

• Complement: Liver-produced proteins activated via classical, lectin, or alternative pathways; effects include cell lysis, enhanced inflammation, virus neutralization, and phagocytosis.

• Cytokines: Proteins (tumor necrosis factor, interferon, interleukins) that regulate immune responses.

• Inflammatory Response: Vasodilation, increased permeability, pain, chemotaxis, and phagocyte recruitment.

• Fever: Pyrogens act on hypothalamus to raise body temperature, increasing metabolism and pathogen destruction.

Module 20.4 Adaptive Immunity: Cell-Mediated Immunity

Adaptive immunity is characterized by specific responses to antigens, primarily through T cells.

Antigens

• Immunogens: Trigger immune responses.

• Self-antigens: Identify self cells to prevent autoimmunity.

• Haptens: Small antigens requiring a protein carrier to elicit a response (e.g., poison ivy).

T Cell Responses

• Maturation: T cells mature in thymus, develop specific receptors, and become immunocompetent.

• Major Histocompatibility Complex (MHC): Glycoproteins presenting antigens to T cells; Class I (endogenous), Class II (exogenous).

• Activation: Dendritic cells present antigens, T cells undergo clonal selection and differentiation into effector and memory cells.

• Helper T Cells: Stimulate macrophages, activate cytotoxic T cells, and stimulate B cells.

• Cytotoxic T Cells: Destroy infected cells via perforin and enzymes.

• Transplantation: Autograft (self), allograft (other person); risk of rejection managed by HLA matching and immunosuppressive therapy.

Module 20.5 Adaptive Immunity: Antibody-Mediated Immunity

B cells mediate antibody production, providing targeted defense against pathogens.

B Cell Maturation and Activation

• Self-reactive B cells: Destroyed to prevent autoimmunity.

• Naïve B cell clones: Mature and reside in spleen and lymph nodes.

• Activation: B cell binds antigen, processes it, presents on MHC, and is activated by helper T cells.

• Clonal Selection: B cells divide into plasma cells (secrete antibodies) and memory B cells (long-lived).

Antibody Structure and Classes

• Constant Region: Structural base.

• Variable Region: Unique, binds specific antigens.

• Antigen Binding Sites: Where antigens attach to antibodies.

• Classes:

  • IgG: Most common, crosses placenta.

  • IgA: Found in secretions (saliva, mucus, tears, breast milk).

  • IgM: Largest, first responder.

  • IgE: Binds to cells with parasitic or environmental pathogens; allergies.

  • IgD: Present on B cells, not secreted.

Functions of Secreted Antibodies

• Neutralization: Prevents toxins/viruses from attaching to cells.

• Agglutination and Precipitation: Antibodies bind multiple antigens, forming complexes for enhanced phagocytosis.

• Activation of Complement: Leads to cell lysis.

• Opsonization: Coats pathogens to enhance phagocyte binding.

• Stimulation of Inflammation: IgE stimulates basophils and mast cells.

Immunological Memory

• Primary Response: Slow, mainly IgM, takes 14 days to peak.

• Secondary Response: Faster, mainly IgG, peaks in 3-5 days, lasts longer.

• Vaccinations: Expose body to antigens, stimulate antibody production and memory cells.

• Active Immunity: Body produces antibodies (infection or vaccination); memory cells formed.

• Passive Immunity: Antibodies administered (placenta, breast milk, injections); no memory cells.

Module 20.7 Disorders of the Immune System

Immune system disorders include hypersensitivity, immunodeficiency, and autoimmune diseases.

Hypersensitivity Disorders

• Type I: Immediate Hypersensitivity: Allergens bind B cells, leading to IgE production and mast cell sensitization; subsequent exposures cause rapid inflammatory responses (anaphylactic shock).

Immunodeficiency Disorders

• SCID: Failure to develop immune response; defective cells or antibodies.

• AIDS: Viral infection targets helper T cells, impairs antibody production.

Autoimmune Disorders

• Multiple Sclerosis: Attacks myelin sheath, impairs nerve transmission.

• Rheumatoid Arthritis: Autoantibodies attack joints.

• Type I Diabetes Mellitus: Autoantibodies attack pancreas, halt insulin secretion.

Additional info: Both stress and aging diminish immune response.