Non-Specific Defences (OCR Gateway GCSE Biology: Combined Science)

• Humans' initial defences against disease include non-specific lines of defence
  • They are called non-specific because they act in the same way against all types of pathogen
  • They include physical and chemical barriers to prevent entry of pathogenic microorganisms

Physical barriers

• Physical barriers – structures that make it difficult for pathogens to enter the body
  
  • Skin - covers almost all parts of the body to prevent infection from pathogens
    • If it is cut or grazed, it immediately begins to heal itself, often by forming a scab
  • Hairs and mucus in the nose and airways
    • These make it difficult for pathogens to get past the facial airways and further into the breathing tract
    • Prevents pathogens from being inhaled into the lungs
  • Mucus and cilia in the trachea and bronchi
    • Pathogens get trapped in mucus produced by cells in the airways of the lungs
    • Other cells lining the trachea and bronchi have cilia (microscopic hair-like structures) that waft mucus up to the back of the throat
    • So it can be excreted (by coughing, blowing the nose, swallowing etc.)

Chemical barriers

• Chemical barriers – substances (produced by specialised cells) that trap or kill pathogens before they can penetrate further into the body and cause disease
  • Stomach acid - contains hydrochloric acid which is strong enough to kill any pathogens that have been caught in mucus in the airways and then swallowed or have been consumed in food or water
  • Lysozymes - enzymes released in tears that break down and kill bacteria on or around the eye surface
  • Natural bacterial flora in the gut and vagina
    • Protect against infection from pathogenic bacteria by outcompeting the pathogen
  • Sebum on the surface of the skin - kills bacterial and fungal pathogens

The different mechanisms can be divided into biochemical and physical defences