Human Defence Responses (Edexcel GCSE Combined Science: Biology): Revision Note

Human Physical Barriers

• The human body has a number of mechanisms that are the first line of defence against an infection

• Physical barriers – structures that make it difficult for pathogens to get past them and into the body

  • Skin - covers almost all parts of your 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 - these make it difficult for pathogens to get past them further up the nose so they are not 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 removed from the body (by coughing, blowing the nose, swallowing etc.)

Human Chemical Barriers

• Chemical barriers – substances produced by the body cells that trap or kill pathogens before they can get 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 produced by the eyes and released in tears will breakdown and kill bacteria on or around the eye

  • 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 first line of defence involves several chemical and physical barriers which prevent pathogens from entering the body