Communicable diseases, disease prevention, the immune system

• Passive defence mechanisms are always present

  • Some of these mechanisms are physical barriers that prevent pathogens from entering
  • Some are chemicals that reduce or prevent the growth of pathogens
• Example of physical barriers:

  • Waxy cuticles
  • Bark
  • Closed stomata
  • Cellulose cell wall
• • Chemical defences prevent pathogens from growing on the surface of the plant by creating acidic conditions
• Examples of chemical defences:

  Toxic compounds

  • Catechol

  Sticky resin found in bark

  • Traps the pathogens so they can't spread
• phagocytosis

  • phagocyte recognises antigens on pathogen
  • membrane and cytoplasm of phagocyte move around the pathogen, engulfing it. This can be made easier by opsonins - molecules in blood that attach to foreign antigens to aid phagocytosis.
  • pathogen now contained in a phagosome (vesicle) in cytoplasm of phagocyte.
  • a lysosome fuses with phagosome to break down pathogen.
  • phagocyte then presents the pathogen's antigens. It sticks the antigens on its surface to activate other immune system cells. Phagocyte is acting as an antigen-presenting cell
• phagocytes activate T lymphocytes

  • their surface is covered with receptors
  • the receptors bind to antigens presented by APCs
  • each T lymphocyte has a different receptor on its surface.
  • when the receptor on the surface meets a complementary antigen, it binds to it. Each T lymphocyte will bind to a different antigen.
  • this activates the T lymphocyte - clonal selection
  • the T lymphocyte undergoes clonal expansion. It divides to produce clones of itself.
• T lymphocytes activate B lymphocytes

  • they're covered with antibodies
  • antibodies bind to antigens to from an antigen-antibody complex.
  • each B lymphocyte has a different shaped antibody on it surface.
  • when the antibody on the surface of a B lymphocyte meets a complementary antigen, it binds to it. Each B lymphocyte binds to a different antigen.
  • This with substances released from T helper cells activate the B lymphocyte. This is also clonal selection.
  • The activated B lymphocyte divides by mitosis into plasma & memory cells. This is also clonal expansion.
• plasma cells male more antibodies to a specific antigen

  • plasma cells are clones of B lymphocyte
  • secrete lot of the antibody specific to the antigen, into blood
  • these antibodies will bind to antigens on surface of pathogen to form lots of antigen-antibody complexes.
• antibodies

  • glycoproteins made of 4 polypeptide chains
  • 2 heavy chains, 2 light chains. Each chain has a variable region & a constant region
  • variable regions of antibody from antigen binding sites
  • shape of variable region is complementary to a particular antigen. Variable regions differ between antibodies.
  • hinge regions allow flexibility when the antibody binds to antigen
  • constant regions allow binding to receptors on immune system cells. It is the same in all antibodies.
  • disulfide bridges hold the peptide chains of the protein together.
• agglutinating pathogens

  • each antibody has 2 binding sites
  • antibody can bind to 2 pathogens at the same time
  • pathogens become clumped together.
  • phagocytes bind to antibodies and phagocytose a lot of pathogens all at once.
  • antibodies that behave in this way are known as agglutinins.
• neutralising toxins

  • toxins have different shapes
  • antibodies called anti-toxins can bind to toxins.
  • this prevents the toxins from affecting human cells
  • toxins are neutralised
  • the toxin-antibody complexes are also phagocytosed.
• Compare and contrast types of immunity

  Artificial active- OFTEN VACCINE human body is stimulated into producing antibodies and memory cells against the antigens in the vaccine. Injects are lab made, so artificial

  Artifical passive- READYMADE antibodies are injected into the body (again the body is not making antibodies itself and so its passive) Artificial as it is an injection

  Natural passive- Transfer of antibodies from the mother to fetus in placenta for fetus to use in its body (so passive)

  Natural active- body is exposed to an infection and will manufacture its own antibodies
• Tuberculosis

  Affects: Animals

  Pathogen responisble: Bacterium
• Bacterial meningitis

  Affects: Humans

  Pathogen responsible: Bacterium
• Ring rot

  Affects: Potatoes, tomatos

  Pathogen responsible: Bacterium
• HIV/AIDS

  Affects: Humans

  Pathogen repsonible: Virus
• Influenza

  Affects: Animals and humans

  Pathogen responsible: Virus
• Tobacco mosaic virus

  Affects: Plants

  Pathogen responsible: Virus
• Black sigatoka

  Affects: Banana plants

  Pathogen responsible: Fungus
• Ringworm

  Affects: cattle

  Pathogen responsible: Fungus
• Athlete's foot

  Affects: Humans

  Pathogen responsible: Fungus
• Potato/ tomato late blight

  Affects: Potato/ tomato

  Pathogen responsible: Protoctist
• Malaria

  Affects: Animals, humans

  Pathogen responsible: Protoctist