Pathogens & Barriers (Cambridge O Level Biology)

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Pathogens

  • A pathogen is a disease-causing organism
  • Pathogens are passed on from one host to another and therefore the diseases they cause are known as transmissible diseases
  • Pathogens can be passed on from host to host in different ways, including:
    • Direct contact - the pathogen is passed directly from one host to another by transfer of body fluids such as blood or semen (eg HIV, gonorrhoea, hepatitis B & C)
    • Indirect contact - the pathogen leaves the host and is carried in some way to another, uninfected individual

Methods of transmission table

Method of Transmission Examples of diseases spread in this way
Droplets in air Common cold, influenza
Food or water Cholera, typhoid, dysentery
Touching contaminated surfaces Athlete's foot, Salmonella (can be transmitted on the feet of flies that land on food that then gets eaten)
Insect bites Malaria, dengue fever

The Body Defences

  • The body has several barriers to the entry of pathogens; these barriers can be either mechanical or chemical
    • Mechanical, or physical, barriers make it difficult for pathogens to get past them and into the body
    • Chemical barriers are substances produced by the body cells that trap or kill pathogens before they can get further into the body and cause disease
  • Examples of barriers to the entry of pathogens include:
    • 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
    • Mucus and hairs in the nose and airways make it difficult for pathogens to get past them and into the airways and lungs
      • Mucus traps pathogens
      • The hairs waft the mucus out of the airways
    • The stomach contains hydrochloric acid which is strong enough to kill any pathogens that are swallowed
  • Other examples include enzymes in tears, and sebum, an oily substance produced by the skin

Examiner Tip

Note that while the body does have other non-specific defences, such as phagocytes, and specific defences, such as antibodies, these mechanisms only come into play once a pathogen has entered the body, so are not barriers to the entry of pathogens

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Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.