Defending against pathogens

• • The human body has features that stop a lot of nasties from entering the blood. 
  • These are non-specific defences (they aren’t produced in response to a particular pathogen) and they’re always present.
  • These can be physical (if they act as a physical barrier to pathogens), chemical (if they involve chemicals which kill pathogens) or microbial (if they involve other microorganisms). 
• PHYSICAL DEFENCES

  • Cilia
  • Skin
• Cilia 

  • The whole respiratory tract (nasal passage, trachea and lungs) is lined with mucus and cilia (hair-like structures)
  • The mucus traps particles that could contain pathogens and the cilia waft the mucus up to the back of the throat where it can be swallowed.
  • Hair in the nose.
• Skin

  • The skin acts as a barrier to pathogens.
• CHEMICAL DEFENCES

  Any natural opening has chemical defences.

  • Lysozyme (Tears)
  • Amylase (saliva)
  • Stomach acid
• Lysozyme (Tears)

  • An enzyme which breaks down bacteria on the surface of the eye.
• Amylase (Saliva)

  • Produced in the mouth it contains molecules which kill pathogens that enter the mouth, so they don't reach the stomach.
• Stomach acid

  • The stomach produces hydrochloric acid that kills pathogens.
• MICROBIAL DEFENCES

  Some pathogens manage to make it past the saliva in the mouth and the acid in the stomach and enter the gut. Here, they have to compete with the bacteria which naturally live in the gut (intestines), in order to survive.

  And even if pathogens do manage to make it past all of these defences to enter the blood, there's still a complex immune system, involving antibodies and immune cells that they have to deal with.

  • Platelets
  • Microbes
• Platelets

  • Platelets in the blood clump together to 'plug' the wound. This is known as blood clotting. 
  • Blood clots stop you from losing too much blood and help to prevent microorganisms from entering the blood.
  • Platelets - are tiny fragments of cells.
  • They contain lots of different substances that are needed to help form the clot. 
  • They also have proteins on their surface which help them stick together and to the site of the wound.
• Microbes

  • are microorganisms that can cause diseases. 

  They can enter the body in a number of ways:

  • They can enter through cuts or bites in the skin or other natural openings
  • They can be breathed in through the mouth or nose

  Microbes (or the poisons they produce) damage cells - these are the symptoms of a disease.