The Lymphatic System and Immunity: Structured Study Notes

The Lymphatic System and Immunity

Lymphatic System Overview

The lymphatic system is a network of cells, tissues, and organs that plays a crucial role in immunity and in returning interstitial fluid to the bloodstream. It is essential for defending the body against infection, illness, and disease.

• Immunity: The ability to defend the body against pathogens and abnormal cells.

• Fluid Balance: Returns interstitial fluid to the bloodstream, preventing tissue swelling.

Components of the Lymphatic System

• Lymphocytes: Primary cells responsible for immune responses. Types include T cells, B cells, and NK cells.

• Lymph: Interstitial fluid that enters lymphatic vessels.

• Lymphatic Vessels (Lymphatics): Begin in peripheral tissues and end at veins, transporting lymph.

• Lymphoid Tissues and Organs: Sites where lymphocytes are formed, mature, or are activated.

Lymphocyte Classes and Functions

• T cells (~80%): Responsible for cell-mediated immunity. Subtypes include:

  • Cytotoxic T cells: Destroy infected or abnormal cells.

  • Helper T cells: Stimulate T and B cell activity; also called regulatory T cells.

  • Suppressor T cells: Inhibit T and B cell activation.

• B cells (10–15%): Responsible for antibody-mediated (humoral) immunity. When activated, they become plasma cells that secrete antibodies.

• NK (Natural Killer) cells (5–10%): Provide immune surveillance by attacking foreign, infected, or cancerous cells.

Antigens are substances (often proteins) that stimulate an immune response. Lymphocytes are sensitive to specific antigens.

Lymphocyte Development and Distribution

• Red Bone Marrow: Produces lymphoid stem cells, which give rise to all lymphocyte types.

• Thymus: Site where some lymphoid stem cells migrate to become T cells, undergoing selection to prevent self-reactivity.

• B cells and NK cells: Mature in bone marrow and migrate to peripheral tissues.

• Lymphocytes: Retain the ability to divide and produce clones, essential for effective immune responses.

Lymphoid Tissues and Organs

Lymphoid Tissues

• Lymphoid Nodules: Densely packed lymphocytes in areolar tissue, often forming clusters (e.g., Peyer's patches in intestines).

• MALT (Mucosa-Associated Lymphoid Tissue): Protects mucosal surfaces in digestive, respiratory, urinary, and reproductive tracts.

Tonsils

• Pharyngeal tonsil (adenoid): Posterior wall of nasopharynx.

• Palatine tonsils: Posterior, inferior margin of oral cavity.

• Lingual tonsils: Base of the tongue.

Lymph Nodes

• Small, bean-shaped organs (1–25 mm) surrounded by a fibrous capsule.

• Located in neck, groin, and axillae.

• Function as filters, removing 99% of pathogens from lymph before it returns to the bloodstream.

Clinical Disorders of Lymphoid Tissues

• Tonsillitis: Inflammation of the tonsils.

• Appendicitis: Inflammation of lymphoid tissue in the appendix.

The Thymus

• Produces thymosins (hormones important for T cell development).

• Largest before puberty (~40 g), shrinks with age (involution), increasing disease susceptibility.

The Spleen

• Largest mass of lymphoid tissue; filters blood like lymph nodes filter lymph.

• Functions:

  • Removes abnormal blood cells by phagocytosis.

  • Stores recycled iron.

  • Initiates immune responses to antigens in blood.

• Contains red pulp (RBCs, macrophages) and white pulp (lymphocytes).

• Fragile; damage may require removal (splenectomy).

Immunity

Types of Immunity

• Innate (Nonspecific) Immunity: Present at birth; provides general defense against pathogens.

• Adaptive (Specific) Immunity: Acquired after exposure; targets specific threats using lymphocytes.

Physical Barriers and Phagocytes

• Physical Barriers: Skin, hair, epithelial linings, and secretions (e.g., sweat, mucus, stomach acid) prevent pathogen entry.

• Phagocytes: Engulf and destroy pathogens and debris. Types include:

  • Neutrophils: Abundant, fast-acting, phagocytize bacteria.

  • Eosinophils: Target antibody-coated pathogens.

  • Macrophages: Derived from monocytes; can be fixed or free.

Adaptive Immunity: Active and Passive

• Active Immunity: Develops after exposure to antigen.

  • Naturally acquired: Through environmental exposure (e.g., infection).

  • Artificially induced: Through vaccination.

• Passive Immunity: Transfer of antibodies from another source.

  • Naturally acquired: Maternal antibodies via placenta or breast milk.

  • Artificially induced: Administration of antibodies (e.g., antivenom).

Properties of Adaptive Immunity

• Specificity: Each lymphocyte targets a specific antigen.

• Versatility: Millions of lymphocytes, each with unique specificity; can clone themselves.

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

• Tolerance: Immune system ignores self-antigens but targets foreign antigens.

Mechanisms of Adaptive Immunity

Antigen Presentation and MHC Proteins

• Antigenic Presentation: Antigens are displayed on cell membranes, triggering immune responses.

• MHC Proteins: Glycoproteins on cell surfaces that present antigens.

  • Class I MHC: On all nucleated cells; present antigens from intracellular pathogens.

  • Class II MHC: On antigen-presenting cells (APCs) like macrophages and dendritic cells; present extracellular antigens.

T Cell Activation

• Antigen Recognition: Inactive T cells bind to specific MHC-antigen complexes.

• CD Markers: Proteins on T cells aiding in antigen recognition.

  • CD8: Recognize Class I MHC (cytotoxic, memory, suppressor T cells).

  • CD4: Recognize Class II MHC (helper T cells).

• Activation Steps for CD8 T Cells:

  1. Antigen recognition (binds Class I MHC with antigen).

  2. Costimulation (additional signals ensure correct activation).

  3. Cell division into cytotoxic, memory, and suppressor T cells.

• Cytotoxic T Cells: Destroy infected/abnormal cells via perforins, apoptosis, or lymphotoxins.

• Memory T Cells: Enable rapid response upon re-exposure.

• Suppressor T Cells: Limit immune response after initial activation.

• Helper T Cells (CD4): Activated by Class II MHC; secrete cytokines to stimulate T and B cells.

B Cell Activation and Antibody Production

• Sensitization: B cell binds antigen, internalizes it, and presents it on Class II MHC.

• Activation: Requires helper T cell interaction and cytokine stimulation.

• Differentiation: B cells divide into:

  • Memory B Cells: Remain inactive until re-exposure.

  • Plasma Cells: Secrete large quantities of antibodies.

Antibody Structure and Function

• Composed of two heavy and two light polypeptide chains.

• Constant segments: Form the base; Variable segments: Form antigen-binding sites.

• Antigen-Antibody Complex: Antibody binds to specific antigenic determinant sites.

• Types of Antigens:

  • Complete antigens: Have multiple determinant sites; fully activate B cells.

  • Partial antigens (haptens): Require carrier molecules to elicit a response; can cause allergies.

Classes of Antibodies (Immunoglobulins, Igs)

Class	Function/Location
IgG	Most abundant (80%); resistance to viruses, bacteria, toxins
IgE	Attaches to basophils and mast cells; involved in allergic responses
IgD	On B cell surfaces; role in sensitization
IgM	First antibody produced; anti-A and anti-B antibodies
IgA	In glandular secretions (mucus, saliva, tears, semen); protects mucosal surfaces

Antibody Responses

• Primary Response: Slow; antibody titer peaks 1–2 weeks after first exposure.

• Secondary Response: Rapid and stronger due to memory cells; can last years.

Example: Vaccination

Vaccines introduce antigens to stimulate the primary response, creating memory cells for faster secondary responses upon real exposure.

Additional info: The notes above expand on the original content by providing definitions, examples, and a structured summary of the lymphatic system and immunity, suitable for college-level study.