Non-specific Immune Responses (Edexcel A (SNAB) A Level Biology)

• 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.