Edexcel A (SNAB) A Level Biology:复习笔记6.2.2 Entry Routes of Pathogens

• In order for pathogens to cause disease they need to enter the body
• There are four main ways in which pathogens can enter
  • Broken skin
    • This provides direct access to the tissues and bloodstream
  • The digestive system
    • When we consume contaminated food and drink
  • The respiratory system
    • Every time we inhale
  • Mucosal surfaces
    • The lining of body cavities e.g. inside of nose, mouth, genitals

Barriers to infection

• Skin
  • This provides a physical barrier against infection
  • If the skin is damaged it leaves the exposed tissue beneath vulnerable to pathogens
  • The blood clotting mechanism of the body plays an important role in preventing pathogen entry in the case of damage to the skin
  • Blood clotting takes time, however, so a few pathogens may still enter before a clot forms
• Microorganisms of the gut and skin
  • Collectively these harmless microorganisms are known as the gut or skin flora
  • They compete with pathogens for resources, thereby limiting their numbers and therefore their ability to infect the body
• Stomach acid
  • The hydrochloric acid that makes up a large part of the gastric juices in the stomach creates an acidic environment that is unfavourable to many pathogens present on food and drink
  • Sometimes a few of these pathogens may survive and make their way to the intestines where they infect the gut wall cells and cause disease
• Lysozyme
  • Secretions of the mucosal surfaces, e.g. tears, saliva, and mucus, contains an enzyme called lysozyme
  • This enzyme will damage bacterial cell walls, causing them to burst, or lyse

The body has physical defences that prevent the entry of pathogens