Edexcel International A Level Biology

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6.8 Non-Specific Immune Responses

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Non-specific Immune Responses

  • 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

Phagocytic white blood cells

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

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Marlene

Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.