11.02 Overview: Immune System

1. Basic Overview of the Immune System

• Purpose: Protect the body from infections and diseases by identifying and eliminating pathogens and abnormal cells.
• Components:
  • Organs:
    • Thymus (site of T cell maturation)
    • Spleen (filters blood, removes old red blood cells and pathogens)
    • Lymph Nodes (filter lymph and trap pathogens, site of immune cell activation)
    • Bone Marrow (site of white blood cell production)
  • Cells: White blood cells (leukocytes) including lymphocytes (B cells, T cells, NK cells), phagocytes (macrophages, neutrophils), and others.
  • Molecules:
    • Antibodies (proteins that bind to specific antigens to neutralize pathogens and mark them for destruction)
    • Cytokines (signaling molecules that mediate and regulate immunity, inflammation, and hematopoiesis)
    • Complement Proteins (a group of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens)

2. The Three Lines of Defense

• The immune system employs a tiered defense strategy, categorized into three lines of defense:

a. First Line of Defense: Physical and Chemical Barriers

• These barriers prevent pathogens from entering the body.

• Physical Barriers:
  • Skin: Acts as a physical shield; keratinized layers prevent pathogen entry.
  • Mucous Membranes: Line respiratory, digestive, and urogenital tracts; trap pathogens with mucus.
  • Cilia: Hair-like structures in respiratory tract that move mucus and trapped pathogens out.
• Chemical Barriers:
  • Sebum and Sweat: Produce an acidic environment on the skin, inhibiting microbial growth.
  • Lysozyme: Enzyme in tears, saliva, and mucus that breaks down bacterial cell walls.
  • Stomach Acid: Low pH kills ingested pathogens.

b. Second Line of Defense: Innate Immune Response

• Activated when pathogens breach the first line of defense. It is non-specific and provides an immediate response.

• Key Components:
  • Phagocytes: Engulf and digest pathogens (e.g., macrophages, neutrophils).
  • Natural Killer (NK) Cells: Destroy virus-infected and tumor cells.
  • Inflammatory Response: Involves vasodilation, increased vascular permeability, and recruitment of immune cells to infection sites.
  • Fever: Raises body temperature to inhibit pathogen growth.
  • Complement System: Series of proteins that enhance (complement) the ability of antibodies and phagocytic cells to clear pathogens.

c. Third Line of Defense: Adaptive Immune Response

• Engaged when the innate response is insufficient. It is specific to the pathogen and has memory for faster responses upon re-exposure.

• Key Features:
  • Specificity: Targets specific antigens unique to each pathogen.
  • Memory: Remembers previous encounters for quicker future responses.
  • Diversity: Capable of recognizing a vast array of antigens.

3. Origin of White Blood Cells (Leukocytes)

• All white blood cells originate from hematopoietic stem cells in the bone marrow through a process called hematopoiesis.
• Hematopoietic stem cells are multipotent, allowing differentiation into various blood cells.
• Migration: Differentiated white blood cells enter the bloodstream and migrate to various tissues and organs to perform immune functions.

4. Innate Immune Cells

• The innate immune system provides the first line of cellular defense and responds rapidly in a non-specific manner.

a. Phagocytes

• Macrophages
  • Function: Engulf and digest pathogens and dead cells through phagocytosis.
  • Location: Found throughout tissues, especially in the liver (Kupffer cells), lungs (alveolar macrophages), and brain (microglia).
  • Additional Role: Activate the adaptive immune system by presenting antigens to T cells.
• Neutrophils
  • Function: Rapidly respond to infection sites to phagocytose bacteria and fungi.
  • Characteristics: Short-lived and highly mobile, forming pus during infections.
• Dendritic Cells
  • Function: Capture antigens and migrate to lymph nodes to present them to T cells, bridging innate and adaptive immunity.
  • Location: Found in tissues that are in contact with the external environment, such as the skin (Langerhans cells) and mucosal surfaces.

b. Natural Killer (NK) Cells

• Function: Detect and destroy virus-infected cells and tumor cells without prior sensitization.
• Mechanism: Release cytotoxic granules that induce apoptosis (cell death) in target cells.

c. Mast Cells

• Function: Release histamine and other mediators during allergic reactions and inflammation.
• Location: Present in connective tissues and mucosal surfaces.

d. Eosinophils and Basophils

• Function: Combat multicellular parasites and contribute to allergic responses.
  • Eosinophils: Specifically target parasitic infections.
  • Basophils: Release histamine during allergic reactions.

5. Adaptive Immune Cells

• The adaptive immune system provides a targeted response to specific pathogens and retains memory for more efficient responses upon re-exposure.

a. Lymphocytes

B Cells

• Function: Produce antibodies (immunoglobulins) that neutralize pathogens and mark them for destruction.
• Types:
  • Plasma Cells: Actively secrete antibodies.
  • Memory B Cells: Remain in the body to provide long-term immunity.

T Cells

• Helper T Cells (CD4⁺)
  • Function: Coordinate the immune response by activating other immune cells, including B cells and cytotoxic T cells.
  • Subtypes: Th1, Th2, Th17, each regulating different aspects of the immune response.
• Cytotoxic T Cells (CD8⁺)
  • Function: Directly kill virus-infected cells, tumor cells, and sometimes transplanted cells.
  • Mechanism: Release perforins and granzymes to induce apoptosis in target cells.
• Regulatory T Cells
  • Function: Suppress and regulate the immune response to maintain tolerance and prevent autoimmune diseases.

b. Memory Cells

• Function: Retain information about specific pathogens for a faster and more effective response upon subsequent exposures.
• Types: Memory B cells and memory T cells.

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