M4:S1 Disease and the immune system

• Pathogen

  An organism that causes disease
• Communicable disease

  A disease that can spread between organisms
• Types of pathogen

  • Bacteria
  • Viruses
  • Fungi
  • Protoctista
• Disease is a condition that impairs the normal functioning of an organism
• Both plants and animals can get diseases
• Communicable diseases in the table

  • Tuberculosis
  • Meningitis
  • Ring rot
  • HIV/AIDS
  • Influenza
  • Tobacco mosaic virus
  • Black sigatoka
  • Ringworm
  • Athlete's foot
  • Potato/tomato late blight
  • Malaria
• Direct transmission

  A disease is transmitted directly from one organism to another
• Direct transmission methods

  • Droplet infection
  • Sexual intercourse
  • Touching an infected organism
• Indirect transmission

  A disease is transmitted from one organism to another via an intermediate
• Intermediates for indirect transmission

  • Air
  • Water
  • Food
  • Another organism (vector)
• Spores are the cells that some organisms use to reproduce asexually, including some protoctista and all fungi
• Overcrowded living conditions

  Increase the transmission of many communicable diseases
• Wet summers

  Increase the spread of potato/tomato late blight
• Tropical, humid and hot climate


  Ideal conditions for mosquitoes (malaria vectors) to breed
• Limited access to good healthcare and health education

  Increase the transmission of HIV
• A communicable disease is a disease that can spread between organisms
• Tobacco mosaic virus is a virus that affects plants
• Malaria is caused by a protoctist pathogen
• Diseases can be transferred directly through methods like droplet infection or touching an infected organism
• Climate can affect the spread of diseases, like potato/tomato late blight being more common in wet summers
• The tobacco mosaic virus is usually transmitted between garden plants via gardeners' hands and tools
• Direct vs indirect disease transmission

  Direct is when a disease is transmitted directly from one organism to another, indirect is when it is transmitted via an intermediate
• Tuberculosis is caused by a bacterial pathogen
• Pathogens need to enter an organism to cause disease
• Primary, non-specific animal defences

  • Skin as a physical and chemical barrier
  • Mucous membranes trapping pathogens
  • Blood clotting to prevent entry
  • Inflammation to isolate pathogens
  • Wound repair to re-form barrier
  • Expulsive reflexes like coughing and sneezing
• If pathogens make it past primary defences, the animal's immune system will respond
• Plant physical defences

  • Waxy cuticle on leaves/stems
  • Cell walls surrounding plant cells
  • Callose deposition to limit pathogen entry and spread
• Plant chemical defences


  • Antimicrobial chemicals like saponins and phytoalexins
  • Chemicals toxic to insect vectors
• Damaged plant cells are more susceptible to infection by the tobacco mosaic virus than normal cells
• Antigen

  Molecules, usually proteins or polysaccharides, found on the surface of cells
• Immune response

  Specific and non-specific stages triggered by foreign antigens
• Phagocytosis by phagocytes

  • Phagocyte recognises antigens
  • Phagocyte engulfs pathogen
  • Phagosome forms
  • Lysosome fuses and digests pathogen
  • Phagocyte presents pathogen antigens
• Neutrophils

  • First white blood cells to respond to a pathogen, move towards wound in response to cytokines
• T lymphocytes

  • White blood cells with receptors that bind to antigens presented by antigen-presenting cells
• Ways to hide negative charges on pathogen membrane

  • Making it easier for negatively charged phagocyte to get closer to the pathogen
• Phagocyte presenting pathogen antigens

  1. Sticks antigens on its surface to activate other immune system cells

  2. Acting as an antigen-presenting cell (APC)
• Neutrophils

  A type of phagocyte, the first white blood cells to respond to a pathogen inside the body
• Neutrophils responding to a wound

  Move towards the wound in response to signals from cytokines released by cells at the site of the wound
• T lymphocyte

  A type of white blood cell with receptors on its surface
• T lymphocyte activation

  1. Receptor on T lymphocyte binds to complementary antigen presented by APC

  2. This activates the T lymphocyte, a process called clonal selection

  3. The T lymphocyte then undergoes clonal expansion, dividing to produce clones of itself