Lesson 17: Immune System

Notes on Immune System

1. Human Body's Innate Defenses

• Immunity is the body's ability to recognize, neutralize, or destroy harmful foreign substances (pathogens: viruses, bacteria, fungi, parasites, prions).

• The immune system consists of innate immunity (fast, general response) and adaptive immunity (specific, slower response with memory).

• Physical barriers: Skin (acidic secretions, lysozymes), mucosal membranes (mucus trapping pathogens), tears, saliva, stomach acid.

• Innate immune response: Recognizes general pathogen features via receptors, triggering immune actions such as cytokine signaling, antimicrobial peptides, complement system activation, and inflammation.

• White blood cells (leukocytes):

  • Natural killer (NK) cells: Kill virus-infected host cells.

  • Macrophages & Neutrophils: Engulf and destroy pathogens via phagocytosis.

  • Eosinophils: Release toxins to kill parasites.

  • Basophils & Mast cells: Release histamine to trigger inflammation.

  • Dendritic cells: Link innate and adaptive immunity by presenting antigens to adaptive immune cells.

2. Inflammatory Response

• Triggered by histamine release from damaged cells.

• Causes vasodilation, increasing blood flow (redness, heat).

• Increases capillary permeability, leading to swelling and pain.

• Fever: Caused by cytokine interleukin-1, which raises body temperature to slow pathogen growth and enhance immune function.

• Recruits immune cells like neutrophils and macrophages to the infection site.

3. Lymphatic System Organs & Their Roles

• Primary lymphoid organs:

  • Bone marrow: Produces B-cells and T-cells; B-cells mature here.

  • Thymus: T-cells mature here.

• Secondary lymphoid organs:

  • Lymph nodes & spleen: Sites where mature lymphocytes encounter pathogens.

  • Mucosa-associated lymphoid tissue (MALT): Includes tonsils, appendix, and gut-associated immune cells.

  • Antigen-presenting cells (APCs): Dendritic cells and macrophages present antigens to adaptive immune cells in lymphoid organs.

4. Adaptive Immune Response: Humoral vs. Cellular Immunity

• Humoral Immunity (B-cells):

  • B-cells recognize intact antigens on pathogens.

  • Activated B-cells undergo clonal expansion, differentiating into plasma cells (which secrete antibodies) and memory B-cells (for faster future response).

• Cell-mediated Immunity (T-cells):

  • Cytotoxic T-cells (CD8+): Destroy infected or cancerous cells presenting non-self antigens.

  • Helper T-cells (CD4+): Do not kill directly but activate B-cells and macrophages via cytokine release.

  • Major histocompatibility complex (MHC) proteins:

    • MHC-I: Found on all nucleated cells; presents to cytotoxic T-cells.

    • MHC-II: Found on APCs; presents to helper T-cells.

5. Antibody Structure & Function

• Antibodies (Immunoglobulins, Ig): Y-shaped proteins with variable regions (specific antigen binding) and constant regions (function classification).

• Types of antibodies:

  • IgG: Most abundant, provides long-term immunity.

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

  • IgM: First produced in infections.

  • IgE: Involved in allergic reactions.

  • IgD: Function not fully understood, involved in B-cell activation.

• Functions of antibodies:

  • Neutralization: Binds to pathogens/toxins to block their activity.

  • Opsonization: Marks pathogens for phagocytosis.

  • Complement activation: Triggers the complement system to destroy pathogens.

6. Immune System Malfunctions

• Autoimmune diseases: The immune system mistakenly attacks self (e.g., lupus, multiple sclerosis, rheumatoid arthritis).

• Allergies: Overreaction to harmless substances (e.g., pollen, food, dust).

• Immunodeficiency disorders:

  • Primary (genetic): SCID (severe combined immunodeficiency).

  • Secondary (acquired): HIV/AIDS destroys helper T-cells, leading to immune failure.

• Hypersensitivity: An excessive immune response causing tissue damage.

Summary

• The immune system consists of innate (nonspecific) and adaptive (specific) immunity.

• The inflammatory response recruits immune cells to infection sites and triggers fever.

• The lymphatic system produces and supports immune cells.

• Humoral immunity (B-cells and antibodies) and cell-mediated immunity (T-cells) work together to defend against pathogens.

• Antibodies play a crucial role in pathogen neutralization and immune signaling.

• Malfunctions in the immune system can lead to autoimmune diseases, allergies, or immunodeficiency disorders.