Innate Immunity and Infection Response Notes

Overview of Innate Immunity

  • Innate immunity is the body's first line of defense against pathogens, consisting of mechanical, chemical, and cellular components.

Infection and Immunity

  • Our bodies face constant microbial attacks.
  • The evolution of immune responses includes:
    1. Physical Barriers: Act as the first line of defense.
    2. Non-Specific Protective Cells and Chemicals: Respond quickly to infections.
    3. Specific Lymphocyte Killing: Target specific pathogens.
    4. Long-Lived Lymphocytes: (Memory cells) that provide lasting immunity.

Components of Innate Immunity

  • Physical Barriers:
    • Skin: A tough barrier made of dead cells, strengthened by collagen and hyaluronic acid. Secretes salty sweat and sebum (low pH ~5).
    • Mucosal Membranes: Line body cavities open to the environment. Produce mucous to trap pathogens and are equipped with cilia to expel these pathogens.
  • Inflammatory Responses: Non-specific and initiated by tissue damage, resulting in recruitment of immune cells and other factors to the infection site.
  • White Blood Cells (Leukocytes): Combat infections through various mechanisms.
    • Neutrophils: First responders, conducting phagocytosis and releasing toxins.
    • Macrophages and Dendritic Cells: Phagocytic cells that also present antigens to activate adaptive immunity.

Inflammatory Response

  • Triggered by tissue damage signaling through cytokines, leading to enhancement of immune activity at the injury site.
  • Cytokines:
    • Pro-inflammatory: Stimulate the immune response (e.g., histamine leading to vasodilation).
    • Anti-inflammatory: Suppress excessive responses to avoid chronic inflammation.
    • Pyrogens: Induce fever to optimize immune function and inhibit pathogen growth.

Challenges in Pathogen Defense

  • Different pathogens (especially Gram-negative bacteria) exploit weaknesses in the immune response, often causing severe illnesses due to their lipopolysaccharides (Lipid A).

Antigen Presentation and Activation of Adaptive Immunity

  • Antigens are molecules that trigger an immune response; specific lymphocytes detect and respond to them, linking innate and adaptive immunity.
    • Antigen Presenting Cells (APCs): Present antigens to T cells, promoting their activation.

Adaptive Immunity

  • Developed as a response to specific pathogens, characterized by:
    • Specificity: Targets specific antigens.
    • Inducibility: Limits activation to the presence of specific pathogens.
    • Clonality: Rapid replication of specific lymphocytes upon activation.
    • Unresponsiveness to Self: Prevents attacking body's own cells.
    • Memory: Long-term immunity through memory cells for quicker responses upon re-exposure.

Lymphocyte Types

  • B Cells: responsible for the Humoral response producing antibodies.
  • T Cells: involved in Cell-mediated immunity, differentiated into CD4+ (helper) and CD8+ (cytotoxic).

HIV and the Immune System

  • HIV targets CD4+ T cells, crucial for the immune system, leading to severe immunosuppression and increased infection risk.

Clinical Implications of Innate and Adaptive Immunity

  • Evaluating white blood cell counts can provide insight into underlying infections or immune disorders.
  • Patterns in leukocyte types can indicate specific infections (e.g., viral vs bacterial).

Summary

  • Innate immunity offers immediate defense, while adaptive immunity provides targeted, long-lasting protection. Both systems are essential for a robust immune response.