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.
