1.1 defensive barriers

epithelial surfaces

• Respiratory tract
• Skin
• Alimentary tract
• Urogenital tracts

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)

Mucous

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

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)

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

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

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)