4.1.1 Communicable diseases

• What are the two types of phagocyte?

  Neutrophils and macrophages.
• phagocytosis

  1. phagocyte recognises antigens on pathogen
  2. 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.
  3. pathogen now contained in a phagosome (vesicle) in cytoplasm of phagocyte.
  4. a lysosome fuses with phagosome to break down pathogen.
  5. 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
• neutrophils

  • first WBC to respond to a pathogen inside the body.
  • neutrophils move towards a wound in response to signals from cytokines (proteins that act as messenger molecules).
  • cytokines are released by cells at the site of the wound.
• 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 make 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.
• 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 the pathogen can't attach to or infect the host cells.
• When a pathogen enters the body for the first time, the antigens on its surface activate the immune system
• The immune response is slow initially because there aren't many B lymphocytes that can produce the necessary antibody
• The body eventually produces enough antibodies to overcome the infection, but the person will show symptoms in the meantime
• After exposure to an antigen, both T and B lymphocytes produce memory cells that remain in the body for a long time
• Memory T lymphocytes remember the specific antigen and will recognize it
• Memory B lymphocytes record the specific antibodies needed to bind to the antigen
• The presence of memory cells means that the person is now immune to that specific pathogen
• Define the term parasite [3]

  • lives in / on host
  • gains nutrition from host
  • at the expense of host
• bacterial diseases


  • tuberculosis
  • bacterial meningitis
  • ring rot
• viral diseases

  • HIV/AIDS
  • influenza
  • TMV
• protoctist diseases


  • Malaria
  • tomato/potato blight
• fungal diseases


  • black sigatoka
  • ringworm
  • athlete's foot
• neutrophil specialisations


  • specific receptors
  • well-developed cytoskeleton
  • many lysosomes
  • many mitochondria
  • lobed nucleus
• Outline the processes that lead to the production of antibodies against an unfamiliar bacterium.

  • B cells have antigen receptor on surface complementary to only one antigen
  • activated B cell divides by mitosis
  • differentiates into plasma cells
  • which secrete antibodies specific to antigen
• autoimmune disease

  abnormal immune response (1)

  against tissues normally in the body (1)