11. Innate immunity. The protective role of skin, mucous membranes, organs and normal microbial flora. Humoral factors of innate immunity – lysozyme, complement system, interferons and cytokines. Cellular factors of innate immunity. Phagocytosis. Inflammation.

Natural Resistance (Innate Immunity)

• Natural Resistance (or Innate Immunity) is the immunity that an individual has due to their genetic makeup.
• It is the immunity that is quick to respond and is present/active from birth.
• Innate immunity is the non-specific, immediate response to pathogens.
• It lacks immunologic memory, is short-lasting, and does not improve upon subsequent exposures.
• It comprises physical, chemical, molecular, and cellular defences.
• Major components include skin and mucous membranes, phagocytic leukocytes (neutrophils and macrophages), specialised lymphocytes (NK cells), and several plasma proteins including proteins of the complement system.
• It can distinguish between self and non-self.
• The response is rapid and prevents the establishment of infection.
• Innate mechanisms are the primary immune defenses.

Protective Role of Skin and Mucosas (Physical Barriers)

• The skin and mucous membranes act as barriers, preventing bacteria from entering the body.
• They are the main factors in the first line of defence against infection.
• These anatomical structures attempt to prevent microorganisms from colonizing the skin and mucosa.
• Skin:
• Acts as a mechanical barrier, with keratin and an acidic environment (pH 3-5), deterring microbial invasion.
• Secretions like sweat, flushing, skin shedding, and organic acids provide additional protection.
• Sebaceous glands produce fatty acids that are bactericidal.
• Secretes antimicrobial proteins like lysozyme and psoriasin.
• Mucous Membranes:
• Contain normal microbiota, which competes with pathogens.
• Mucus traps microorganisms, and cilia help expel them.
• Specific examples:
• Gastrointestinal tract: Peristalsis, gastric acid (low pH), bile acids, digestive enzymes, and gut flora provide protection. Peristalsis is a physiological defence mechanism.
• Respiratory airways and lungs: Mucociliary elevator, surfactant, and defensins. The mucociliary escalator is a physiological defence mechanism.
• Nasopharynx: Mucus, saliva, lysozyme.
• Eyes: Tears. Tears are also a physiological defence mechanism.
• Oral cavity: Lysozyme.