In-Depth Notes on Innate Immunity and Nonspecific Defenses of the Host

Overview of Immunity

• Immunity: Ability to ward off disease.
• Susceptibility: Lack of resistance to a disease.
• Types of Immunity:
  • Innate Immunity: Rapid, nonspecific defenses present at birth.
  • Adaptive Immunity: Slower response to specific pathogens with a memory component.

White Blood Cells (WBCs) in the Immune System

• WBC Count: Normal range 5,000-10,000 WBCs/mm³.
• Differential WBC Count: Indicates the percentages of each WBC type:
  • Neutrophils: 60-70%
  • Lymphocytes: 20-25%
  • Monocytes: 3-8%
  • Eosinophils: 2-4%
  • Basophils: 0.5-1%
• High WBC Count: May indicate infections or autoimmune diseases.
• Low WBC Count: May suggest viral infections or severe bacterial infections.

Innate Immune Cells

• Basophils: Granulocytes that release histamines for inflammation.
• Eosinophils: Granulocytes that kill parasites with oxidative bursts.
• Neutrophils: First responders for bacteria and fungi; perform phagocytosis.
• Monocytes: Differentiate into macrophages; perform phagocytosis and tissue repair.
• Natural Killer (NK) Cells: Kill virus-infected and cancer cells.

Role of Toll-Like Receptors (TLRs)

• TLRs recognize pathogen-associated molecular patterns (PAMPs) from microbes, leading to immune responses.
• TLRs induce cytokine release, regulating the immune response.

Physical and Chemical Factors in Innate Immunity

• Skin as a Barrier: The epidermis and dermis protect against microbial invasion.
  • Skin's dryness and shedding inhibit microbial growth.
• Mucous Membranes: Trap microbes in various tracts (gastrointestinal, respiratory).
• Chemical Defenses:
  • Sebum: Lowers skin pH; antibacterial.
  • Lysozyme: Found in tears and saliva; breaks down bacterial cell walls.
  • Gastric Juice: Acidic environment that destroys pathogens.

Normal Microbiota

• Compete with pathogens through:
  • Competitive exclusion.
  • Producing harmful substances for pathogens.
• Probiotics and prebiotics aid in promoting beneficial bacteria in the gut.

Phagocytosis

• Phagocytosis: Ingestion of microorganisms by phagocytes (e.g., neutrophils, macrophages).
• Mechanism:
  1. Chemotaxis: Phagocytes are attracted by chemical signals.
  2. Adherence: Phagocytes bind to pathogens using TLRs.
  3. Ingestion: Microbes engulfed into phagosomes.
  4. Digestion: Lysosomes fuse with phagosomes to digest microbes using enzymes.

Inflammation

• Definition: Local response to tissue damage (e.g., microbial infection).
• Signs: Pain, redness, immobility, swelling, heat (PRISH).
• Phases:
  • Vasodilation & increased permeability of blood vessels.
  • Migration of phagocytes to infection site.
  • Tissue repair.

Fever as Defense Mechanism

• Elevated body temperature enhances immune response.
• Causes include infections, toxins.
• Benefits include improved phagocytic activity and slowed pathogen growth.

The Complement System

• Composed of 30+ serum proteins enhancing the immune response.
• Classical Pathway: Activated by antibody-antigen complexes.
• Alternative Pathway: Triggered by microbial surfaces.
• Overall functions include opsonization and cytolysis of pathogens.

Interferons

• Proteins released by virus-infected cells to signal neighboring cells to prepare defenses against viral replication.

Antimicrobial Peptides

• Short peptides produced in response to microbial components, inhibiting growth and inducing cell lysis.

Factors Affecting Immune Response

• Genetic resistance can provide advantages (e.g., sickle cell trait).
• Age: Young and elderly are often more vulnerable to infections.
• Hygiene Practices: Proper hygiene can bolster the immune system against infections.