Chapter 20: The Lymphatic System and Immunity

Module 20.1: Structure and Function of the Lymphatic System

The lymphatic system is a vital component of the circulatory and immune systems, responsible for fluid balance, fat absorption, and defense against pathogens.

• Three Functions of the Lymphatic System:

  • Interstitial Fluid Return: Returns excess interstitial fluid from tissues to the bloodstream, preventing edema. 4L a day

  • Transport of Dietary Fats: Absorbs fats and fat-soluble vitamins from the digestive tract and transports into the small intestine.

  • Immunity: Provides defense against pathogens through lymphocytes and other immune cells., Houses white blood cells

• Lymph vs. Lymphatic Vessels, Tissue, and Organs:

  • Lymph: Clear fluid similar to plasma, containing white blood cells, especially lymphocytes.

  • Lymphatic Vessels/ Lymphatics: Network of vessels that transport lymph throughout the body. (thin walls a little bit of muscle, and large diameter, 1 way valves)

  • Lymphatic Tissue: Reticulocytes- reticular fibers (Soft flexible skeleton) , Macrophages: Eat debris/ clean lymph...present to B cells, T cells)

  • Lymphatic Organs: Structures such as lymph nodes (Capsule around several nodules), spleen, and thymus that house immune cells.

• Comparison: Lymphatic Vessels vs. Blood Vessels

  • Lymphatic vessels have thinner walls, more valves, and are not part of a closed circuit like blood vessels.

  • Lymph is moved by skeletal muscle contraction, respiratory movements, and valves preventing backflow (no central pump).

• Lymph Circulation:

  • Lymph flows from lymphatic capillaries → collecting vessels → lymphatic trunks → lymphatic ducts.

  • Lymph re-enters the bloodstream at the junction of the internal jugular and subclavian veins (right lymphatic duct and thoracic duct).

• Lymphatic Tissue Structure and Function:

  • Lymphocytes: Main immune cells (B cells and T cells) that recognize and respond to antigens.

  • Macrophages: Phagocytic cells that engulf pathogens and present antigens.

  • Dendritic Cells: Antigen- eat pathogens presenting cells that activate T cells, B cells.

  • Reticular Cells: Produce reticular fibers that form the structural framework of lymphatic tissue.

  • Lymphoid Follicle/Nodule: Spherical clusters of lymphocytes, often found in lymph nodes and MALT.

  • MALT (Mucosa-Associated Lymphatic Tissue): Lymphatic tissue in mucous membranes; includes tonsils (pharynx), Peyer's patches (small intestine), and appendix (large intestine).

• Lymphoid Organs:

  • Lymph Nodes: Bean-shaped capsule that surrounds lymphoid follicles and house lymphocytes and macrophages.( exits from 1 efferent lymphatic vessel. Germinal centers have naive lymphocytes

  • Spleen: Largest lymphatic organ; recycles RBC's, red pulp filters blood and removes old erythrocytes, white pulp contains lymphocytes for immune response.

  • Thymus: Site of T cell maturation; prominent in children, shrinks with age.

Module 20.2: Overview of the Immune System

The immune system protects the body from pathogens through innate (nonspecific) and adaptive (specific) mechanisms.

• Innate Immunity: Present at birth, provides immediate, nonspecific immunity defense against any pathogens (e.g., skin, phagocytes).

• Adaptive Immunity: Develops after exposure to antigens, is specific immunity to particular pathogens, and involves memory cells (B and T lymphocytes).

• Antibody-Mediated Immunity: B cells produce antibodies that neutralize pathogens.

• Cell-Mediated Immunity: T cells directly attack infected or abnormal cells.

• Connection Between Immune and Lymphatic Systems: Lymphatic system transports immune cells and antigens, and lymphoid organs are sites of immune cell activation.

Module 20.3: Innate Immunity – Internal Defenses

Innate immunity includes cellular and chemical defenses that act rapidly against a wide range of pathogens.

• Phagocytic Cells:

  • Macrophages: Engulf and digest pathogens; present antigens to T cells. ( In CT)

  • Neutrophils: Most abundant white blood cells; first responders to infection. ( Bacteria)

  • Eosinophils: Combat parasites and participate in allergic responses.

  • Dendritic Cells: Capture antigens and migrate to lymph nodes to activate T cells. ( Macrophages)

  • Natural Killer (NK) Cells: Destroy virus-infected and cancerous cells by inducing apoptosis.

  • Basophils/ Mast Cells- regulate inflammation

• Complement System: Group of plasma proteins that enhance immune responses; can cause cell lysis (rupture of pathogen cell membranes). (Made by liver)

• Bacterial Lysis- Poke hole in the cell wall

• WBC activity- Inflammation

• Opsonization- protein stuck to surface... Immobilize

• Cytokines: Signaling molecules between cells

  • TNF (Tumor Necrosis Factor): Promotes inflammation and cell death.

  • Interferons: Infected cells will release these to protect nearby cells

  • Interleukins: Cytokines shared between WBCs for communication

• Inflammatory Response:

  • Cardinal Signs: Redness, heat, swelling, pain (sometimes loss of function as a fifth sign).

  • Vasodilation and Increased Permeability: Allow immune cells and proteins to reach the site of injury or infection.

• Fever: Pyrogens reset the hypothalamic thermostat, raising body temperature to inhibit pathogen growth and enhance immune activity.

Module 20.4: Adaptive Immunity – Cell-Mediated Immunity

Cell-mediated immunity involves T cells that recognize and destroy infected or abnormal cells.

• Thymus: Screens developing T cells for immunocompetence (ability to recognize self vs. non-self).

• Helper T Cells (CD4): Coordinate immune responses by activating B cells, cytotoxic T cells, and macrophages.

• Cytotoxic T Cells (CD8): Destroy infected or cancerous cells by inducing apoptosis.

• Antigen Presentation:

  • Endogenous Antigen Presentation: Involves Class I MHC molecules presenting antigens from inside the cell to CD8 T cells.

  • Exogenous Antigen Presentation: Involves Class II MHC molecules on antigen-presenting cells (APCs) presenting antigens from outside the cell to CD4 T cells.

• Pathos= Disease

• Pathogen = " disease generator" living things!!

• Virus= Tiny particles

  • = Need your cells to reproduce

• Bacteria= very small, single cells

  • = asexual reproduction

  • = usually in interstitial spaces

• Fungi= usually infect surfaces

• T Cell Types: Made in bone marrow, Move to thymus gland

  • Naive T Cells: Have not yet encountered their specific antigen.

  • Sensitized T Cells: Have encountered antigen and are proliferating. Activated T Cells: Needs a second signal

  • Effector Cells: Actively respond to antigen (e.g., helper or cytotoxic T cells). DO THEIR JOB

  • Memory T Cells: Stay in lymph nodes

  • Immunological Memory: The ability of the immune system to respond more rapidly and effectively to pathogens that have been encountered previously.

• Helper T Cells- TH Cells ( T- reg) CD4+ cells

• Cytotoxic T Cells- Tc Cells CD8+

• Role in Immune Response: Activated helper T cells stimulate other immune cells; cytotoxic T cells kill infected cells.

Module 20.5: Adaptive Immunity – Antibody-Mediated Immunity

Antibody-mediated (humoral) immunity involves B cells producing antibodies that neutralize pathogens.

• B Cell Activation and Proliferation:

  • Naive B Cell: Has not yet encountered antigen. In Lymph nodes

  • Sensitized B Cell: Has encountered antigen and is proliferating. Activated: Binds to helper T Cells

  • Plasma Cell: Differentiated B cell that secretes antibodies.

  • Memory B Cell: Provides long-term immunity by responding rapidly to future exposures.

• Five Major Classes of Antibodies (Immunoglobulins):

  • IgG: Most abundant; crosses placenta;

  • IgA: Found in secretions (tears, saliva, mucus, breast milk); protects mucosal surfaces.

  • IgM: First antibody produced in primary response; effective at agglutination.

  • IgE: Involved in allergic reactions and defense against parasites.

  • IgD: Functions mainly as a B cell receptor.

• Antibody Functions:

  • Agglutination: Clumping of pathogens for easier removal.

  • Opsonization: Coating pathogens to enhance phagocytosis.

  • Neutralization: Blocking pathogen activity or toxin binding.

• Primary vs. Secondary Immune Responses:

  • Primary Response: First exposure; slower, mainly IgM produced.

  • Secondary Response: Subsequent exposures; faster and stronger, mainly IgG produced.

• Vaccinations: Induce immunity by exposing the immune system to harmless forms of antigens, stimulating memory cell formation.

• Active vs. Passive Immunity:

  • Active Immunity: Body produces its own antibodies (natural: infection; artificial: vaccination).

  • Passive Immunity: Antibodies are transferred from another source (natural: maternal antibodies; artificial: antibody injection).

Module 20.7: Disorders of the Immune System

Immune system disorders can result from overactivity, underactivity, or misdirected immune responses.

• Hypersensitivity: Excessive immune response to harmless antigens (e.g., allergies).

• Immunodeficiency ( Hyposensitive): Inadequate immune response Secondary disease or virsu(e.g., HIV/AIDS).

• Autoimmune Disorders: Immune system attacks the body's own tissues (e.g., lupus, rheumatoid arthritis).

• HIV and AIDS: HIV infects and destroys CD4 (helper T) cells, leading to immunodeficiency and increased susceptibility to infections (AIDS).

Additional info: The above notes expand on the learning objectives by providing definitions, examples, and context for each major concept in the lymphatic and immune systems. This guide is suitable for exam preparation and review.