1.1 defensive barriers

epithelial surfaces

  • Respiratory tract
  • Skin
  • Alimentary tract
  • Urogenital tracts

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why epithelial surfaces are susceptible to environmental factors that can cause disease

  • ​​Epithelial tissue at external surfaces susceptible to environmental insults trauma & infection
  • Mucous membranes particularly susceptible due to thin walls (required for physiological function)

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Mucous

  • Water & electrolytes
  • Glycoproteins & lipids
  • Antimicrobial compounds   * Transferrin   * Lactoferrin   * Lysozyme
  • Antibodies (IgA)
  • Antioxidants   * Superoxide dismutase   * Catalase   * Glutathione peroxidase

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Routes of infection

  • Horizontal transmission   * Direct contact   * Aerosol (e.g. kennel cough)   * Oro-faecal (e.g. handling faeces & infecting feed)   * Vector-bourne (pathogen uses arthropod host to spread from animal to animal, e.g. ticks & lyme disease)
  • Vertical transmission   * Direct contact (pathogen crosses placental barrier to infect foetus, e.g. bovine virus diarrhoea - BVD)

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adaptations of skin, respiratory tract & alimentary tract

Defensive barriers

Physical barriers - skin

  • epithelial barriers   * Physical - skin   * Physiological - mucous membranes   * Biochemical - gastric acid   * Immunological - macrophages
  • Keratinocytes replicate to generate stratified squamous keratinised epithelium, has receptors binding to microbes, causing signalling cascade, leading to production of various soluble factors (w/ antimicrobial properties)
  • TRIGGERING INFLAMMATION - produce cytokines (immunological hormones) & chemokines (cell migration factors)
  • Melanocytes protect from UV damage
  • Langerhans cells - tissue-resident macrophages present in all layers of epidermis
  • Sebaceous glands & sebum - water repellent, contains fatty acids & has anti-microbial components

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Respiratory tract

  • Mucociliary escalator   * Continual movement of cilia stops microbes from forming colonies by moving them continually   * Host become immunocompromised without this - can be caused by toxic compounds, potentially causing pneumonia   * Viral infection, trauma (from dust) and toxins (ammonia) can occur if mucociliary escalator is compromised
  • Alveolar macrophages   * Main line of defence against inhaled microbial pathogens & inanimate particles   * Derived from blood monocytes & interstitial macrophages   * Short lifespan in alveoli
The respiratory system as a defensive barrier
  • Conductive system:   * Nasal cavity   * Pharnyx   * Larynx   * Trachea   * Bronchi
  • Transitional system   * Bronchioles
  • Gas exchange system   * Alveoli

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Alimentary tract

  • Oral cavity & oesophagus   * Function - prehension, mastication & digestion (salivary enzymes)   * Protection needed against trauma & infectious agents   * Physical barrier - stratified squamous epithelium   * Biochemical barrier - lysozyme in saliva
  • Stomach   * Sterilisation & digestion of food     * HCl acid (pH 2)     * Pepsin (protease)     * Mucous (protect epithelium)     * Regulatory peptides
  • Small intestine   * Cryptidins (similar to denfensins)     * Toxic to bacteria     * Synthesised by Paneth cells present in intestinal crypts
  • Immune defence of GIT   * Gut-associated lymphoid tissue - produces immune responses   * Specific mucosal protective antibody (IgA)

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