• There are two types of immune response in the body once a pathogen enters
  • Non-specific
    • This response is the same, regardless of the pathogen that invades the body
  • Specific
    • This is a response specific to a particular pathogen
    • The immune system is able to recognise specific pathogens due to the presence of?antigens?on their cell surface
      • Antigens are molecules such as?proteins?or?glycoproteins?located on the surface of cells; their role is to act as an ID tag,?identifying a cell as being 'self' or 'non-self'
        • Pathogens have?non-self antigens, so the immune system recognises them as?not belonging to the body
• When a pathogen invades tissue the non-specific immune response begins immediately; this includes
  • Inflammation
  • Interferons
  • Phagocytosis

Inflammation

• The surrounding area of a wound can sometimes become?swollen, warm and painful?to touch; this is?inflammation
• Body cells called?mast cells?respond to tissue damage by secreting the molecule?histamine
  • Histamine is a?chemical signalling molecule?that enables?cell signalling, or communication between cells
• Histamine stimulates the following responses
  • Vasodilation?increases blood flow through capillaries
  • Capillary walls become 'leaky', or?more permeable, allowing fluid to enter the tissues and creating swelling
    • Some plasma proteins leave the blood when the capillaries become more permeable
  • Phagocytes?leave the blood and enter the tissue to?engulf foreign particles
  • Cells release?cytokines, another cell signalling molecule that triggers an immune response in the infected area

Interferons

• Cells infected by viruses produce?anti-viral proteins?called?interferons
• Interferons?prevent viruses from spreading?to uninfected cells
  • They?inhibit?the production of viral proteins, preventing the virus from replicating
  • They?activate?white blood cells involved with the?specific immune response?to destroy infected cells
  • They?increase the non-specific immune response?e.g. by promoting inflammation

Phagocytosis

• Phagocytes?are a type of white blood cell?responsible for?removing dead cells and invasive microorganisms; they do this by engulfing and digesting them
  • The process of engulfing and digesting is known as?phagocytosis
• Phagocytes travel throughout the body and can leave the blood by?squeezing through capillary walls
• During an infection they are released in?large numbers
• Mode of action
  • Chemicals?released by pathogens, as well as chemicals released by the body cells under attack, e.g. histamine,?attract phagocytes?to the site where the pathogens are located
  • They move towards pathogens and?recognise?the antigens?on the surface of the pathogen as being?non-self
  • The?cell surface membrane?of a phagocyte extends out and around the pathogen,?engulfing it?and trapping the pathogen within a?phagocytic vacuole
    • This part of the process is known as?endocytosis
  • Enzymes are released into the phagocytic vacuole when?lysosomes?fuse?with it
  • These digestive enzymes, which includes lysozyme,?digest?the pathogen
  • After digesting the pathogen, the phagocyte will?present the antigens?of the pathogen on its cell surface membrane
    • The phagocyte becomes what is known as an?antigen presenting cell
  • The presentation of antigens?initiates the specific immune response

Phagocytes engulf pathogens in the process of phagocytosis, enclosing them in a phagocytic vacuole. Lysosomes fuse with the vacuole, releasing enzymes which digest the pathogen.