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Table 1 Summary of non-specific host-defense mechanisms for barriers of innate immunity [1].

From: An introduction to immunology and immunopathology

Barrier Mechanism
Anatomic
Skin • Mechanical barrier retards entry of microbes
• Acidic environment (pH 3-5) retards growth of microbes
Mucous membrane • Normal flora compete with microbes for attachment sites
• Mucous entraps foreign microbes
• Cilia propel microbes out of body
Physiologic
Temperature • Body temperature/fever response inhibits growth of some pathogens
Low pH • Acidic pH of stomach kills most undigested microbes
Chemical mediators • Lysozyme cleaves bacterial cell wall
• Interferon induces antiviral defenses in uninfected cells
• Complement lyses microbes or facilitates phagocytosis
Phagocytic/endocytic barriers
  • Various cells internalize (endocytosis) and break down foreign macromolecules
• Specialized cells (blood monocytes, neutrophils, tissue macrophages) internalize (phagocytose), kill and digest whole organisms
Inflammatory barriers
  • Tissue damage and infection induce leakage of vascular fluid containing serum protein with antibacterial activity, leading to influx of phagocytic cells into the affected area