Infection + Immune Responses

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Enter the body through:

  • cuts in the skin
  • the digestive system via contaminated food or drink
  • the respiratory system  by being inhaled
  • mucosal surfaces e.g. the inside of the nose, mouth and genitaIs
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Barriers Preventing Infection

Stomach Acid

  •  acidic conditions of stomach kill pathogens
  • some may survive and pass into intestines where they can invade cells of the gut wall and cause disease


  • acts as a physical barrier
  • if damaged, pathogens on surface can enter bloodstream
  • blood clots at area of damage to prevent pathogens from entering (some may get in before clot froms)
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Barriers Preventing Infection

Gut + Skin Flora

  • intestines + skin are naturally covered in billions of harmless microorganisms (flora)
  • they compete with pathogens for nutrients and space, limiting pathogens lying in the gut and on the skin making it harder to infect the body


  • musocal surfaces produce secretions (tears, saliva, mucus) containing the enzyme lysozyme
  • it kills bacteria by damaging their cell walls, making it burst open (lyse)
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Foreign Antigens

  • antigens are molecules (usually proteins or polysaccharides) found on the surface of cells
  • when pathogens invade the body, antigens on its cell surface are recognised as foreign
  • this activates cells in the immune system
  • the body has two types of respone - specific and non-specific
  • the non-specific response happens in the same way for all microorganisms (it's not antigen specific) and starts attacking the microorganisms straight away
  • non-specific: inflammation, anti-viral proteins, phagocytosis
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Inflammation at the Site of Infection

1) Immune system cells recognise foreign antigens on pathogen surfaces and release molecules that trigger inflammation

2) The molecules cause vasodilation around the site of infection, increasing blood flow to it, as well as increasing the permeability of blood vessels

3) The increased blood flow brings lots of immune system cells to the site of infection and the increased permeability allows those cells to move out of theblood and into the infected tissue

4) Immune system cells can then start to destroy the pathogen

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Anti-Viral Proteins

1) When cells are infected with viruses, they produce anti-viral proteins called interferons

2) Interferons help prevent viruses soreading to uninfected cells

3) They do this by:

  • preventing viral replication by inhibiting the production of viral proteins
  • activating cells involved in the specific immune response to kill infected cells
  • activating other mechanisms of the non-specific immune response e.g. the promote inflammation to bring immune system cells to the site of infection
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1) A phagocyte recognises the antigens on a pathogen

2) The cytoplasm of the phagocyte moves round the pathogen, engulfing it

3) The pathogen is now contained in a phagocytic vacuole (bubble) in the cytoplasm of the phagocyte

4) A lysosome (an organelle containing digestive enzymes) fuses with the phagocytic vacuole, breaking down the pathogen

5) The phagocyte then presents the pathogen's antigens, sticking hem on its surface to activate otheer immune system (it's called an antigen-presenting cell)

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Specific Immune Response - T Cells

  • Phagocytosis activates T cells (a type of white blood cell)
  • Thier surfaces are covered with receptors, which bind to antigens presented by the phagocytes - each cell has different shaped receptors
  • When the recptor on the surface meets a complementary antigen, it binds to it, so each cell will bind to a different antigen
  • This activates the T cell, it divides and differentiates into different types of T cells that carry out different functions:
    • T helper cells release substances to activate B cells, T killer cells, and macrophages
    • T killer cells attach to antigens on a pathogen-infected cell and kill the cell
    • T memory cells divide into the correct type of T cells to kill the cell carrying the antigen
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Specific Immune Response - B Cells

  • B cells are a type of white blood cell covered with proteins called antibodies, which bind to antigens to form an antigen-antibody complex
  • Each B cell has a different shaped antibody on its surface, when it meets a complementary antigen it binds to it (each B cell will bind to a different antigen)
  • This, together with substances released from the T cell, activates the B cell
  • The activated B cell divides by mitosis into plasma cells (also called B effector cells) and B memory cells which produce the right antibody to the antgen
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Plasma Cells

  • Plasma cells are clones of the B cells
  • They secrete lots of the antibody, specific to the antigen, into the blood
  • These antibodies form lots of antigen-antibody complexes:
    • the variable regions of the antibody form the antigen binding sites - the shape of the variable region is complementary to a particular antigen (the variable regions differ between antibodies)
    • the hinge region allows flexibility when the antibody binds to the antigen
    • the constant regions allow binding to receptors on immune system cells (e.g. phagocytes), the constant region is the same in all antibodies
    • disulfide bridges hold the polypeptide chains together
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Antibodies help to clear an infection by:

  • Agglutinatinatin pathogens: each antibody has two binding sites, so an antibody can bind to two pathogens at the same time - the pathogens become clumped together. phagocytes then bind to the antibodies and phagocytose a lot of pathogens all at once
  • Neutralising toxins: antibodies can bind to the toxins produced by pathogens, this prevents the toxins from affecting human cells, so the toxins are neutralised. the toxin-antibody complexes are also phagocytosed
  • Preventing the pathogen binding to human cells:  when antibodies bind to the antigens on pathogens, they may block the cell surface receptors that the pathogens need to bind to the host cells. this means thepathogen can't attach to or infect the host cells
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