The Body's Defense Mechanisms Against Damage and Infection

I. Innate Immune Response

The innate immune system provides the body's first line of defense. It is non-specific and responds quickly to pathogens or injury.

1. Physical Barriers

• Skin: A tough outer layer that blocks pathogen entry. Its acidic pH and natural flora further inhibit microbial growth.
• Mucous Membranes: Line the respiratory, digestive, and urogenital tracts, trapping pathogens and preventing them from entering deeper tissues.

2. Mucous Secretion

• Role: Produced by epithelial cells, mucus traps microbes and particulates.
• Ciliary Action: In the respiratory tract, cilia move mucus and trapped pathogens out of the body (e.g., coughing or sneezing).

3. Chemical Barriers

• Enzymes: Lysozyme in saliva, tears, and mucus breaks down bacterial cell walls.
• Acidic Environments: Stomach acid (pH ~2) kills many ingested pathogens.
• Defensins: Antimicrobial peptides disrupt bacterial membranes.

4. Osmotic Pressure

• Salt Concentration: Sweat and tears create hypertonic conditions that dehydrate and kill microbes.

5. Leukocytes (White Blood Cells)

• Neutrophils: Rapidly engulf and destroy pathogens via phagocytosis.
• Macrophages: Engulf larger particles and release cytokines to recruit other immune cells.
• Natural Killer (NK) Cells: Target and destroy infected or cancerous cells.

II. Adaptive Immune Response

The adaptive immune system provides a specific, slower response but creates long-term immunity.

1. B Lymphocytes (B Cells)
   • Antibody Production: Plasma B cells produce antibodies that neutralize pathogens or mark them for destruction.
   • Memory B Cells: Retain information about specific pathogens for a faster response upon re-exposure.

2. T Lymphocytes (T Cells)

• Helper T Cells (CD4+): Activate B cells, T cells, and macrophages.
• Cytotoxic T Cells (CD8+): Kill infected or abnormal cells by inducing apoptosis.
• Regulatory T Cells: Suppress excessive immune responses to prevent autoimmunity.

3. Blood Clotting

• Role: Prevents further entry of pathogens through wounds and minimizes blood loss.
• Mechanism:
  • Platelets aggregate at the injury site.
  • Clotting factors activate fibrin, forming a stable clot.

4. Inflammation

• Purpose: Localizes and eliminates infection while starting tissue repair.
• Signs: Redness, heat, swelling, pain, and loss of function.
• Phases:
  • Vasodilation increases blood flow.
  • Immune cells migrate to the site (chemotaxis).
  • Cytokines and histamines mediate the response.