Physical and Chemical barriers

Physical and Chemical barriers

  • These are both harnessed by the body to stop pathogens getting in.

Physical barriers

  • Skin: Outer layer skin cells contain keratin which makes the skin very effective at protecting against microorganisms. If damaged, blood clots can seal cuts quickly to keep pathogens out.
  • Respiratory system: The whole respiratory tract is lined with mucus and cilia which are hairs that can catch dust and bacteria before they reach the lungs. The cilia push gunk-filled mucus away from the lungs.

Chemical barriers

  • Oil: Hairs on the skin contain a gland that produces oil called sebum. This kills microorganisms and keeps skin supple and waterproof.
  • Eyes: Eyes produce tears which contain a chemical called lysozyme which kills bacteria on the surface of the eye. Blinking spreads this enzyme over the eyes surface.
  • Stomach: The stomach contains hydrochloric acid that kills any swallowed bacteria in food or mucus.

White blood cells

  • There are two ways white blood cells get rid of pathogens: engulfing and digesting them or killing them with antibodies. Phagocytes engulf the white blood cells and B-Lymphocytes produce antibodies.
  • B-lymphocytes produce molecules called antibodies which remember the shape of the antigen on the pathogen. If a new antigen is found different antibodies are tried until one is found that matches the antigen.
  • The antibody locks onto the antigen and the white blood cell destroys the pathogen. Specific immune response.

Defence against invasion: Tea tree

  • A tea tree is an example of a plant that can produce chemicals to defend themselves. Its leaves produce oil that kills bacteria.
  • Indigenous people of Australia have used these leaves in their medicines for centuries. The purified oil these days is used in many different kinds of antibacterial products such as facial cleansers.


  • These are chemicals that destroy bacteria or stop them growing.
  • They can also be used to prevent pathogens entering an open wound, and around areas with a lot of pathogens (toilet, kitchen)
  • Many household products contain antiseptics such as bathroom cleaners.
  • Hospitals and surgeries use antiseptics to prevent spread of infection such as MRSA.


  • These are drugs used inside the body, usually taken as a pill or injected.
  • They are used to treat patients who already infected with bacteria and fungi.
  • However, they do not destroy viruses.
  • There are two types of antibiotics:
    • Antibacterials: Antibacterial antibiotics are used to treat bacterial infections. They work by killing bacteria or stopping them from growing. However, bacteria can evolve resistance to the antibiotic, meaning it doesn’t work anymore.
    • Antifungals: Antifungal antibiotics such as nystatin are used to treat fungal infections. They work by killing the fungi or stopping them from growing.

Bacteria can become resistant to antibiotics

  • There is always variation within a species so naturally there will be bacteria resistant to antibiotics.
  • When the medicine is first taken the susceptible bacteria are killed first. However, the resistant bacteria will be left behind, especially if the person taking the medicine does not finish their course.
  • The resistant bacteria will survive and reproduce and thus making more resistant bacteria to cause illness. This is an example of natural selection.
  • An example of a resistant strain is MRSA (methicillin-resistant Staphylococcus aureus) which causes serious wound infections.