Module 4

• Communicable diseases are caused by pathogens including bacteria, viruses, protoctista and fungi
• Pathogens cause harm by directly damaging tissue or releasing toxins
• Examples of diseases caused by different pathogens

  • Bacterial diseases: Tuberculosis, Bacterial meningitis, Ring rot
  • Viral diseases: HIV, Influenza, Tobacco mosaic virus
  • Protoctista diseases: Malaria, Potato blight
  • Fungal diseases: Black sigatoka, Ringworm, Athlete's foot
• Prokaryotic cells


  • Classified by shape or cell wall: rods, spherical, comma, spiraled, corkscrew
  • Gram-positive or gram-negative based on cell wall
• Viruses


  • Non-living, acellular, smaller than bacteria, consist of genetic material, capsid, and attachment proteins
  • Can only replicate inside host cells
• Bacteriophages


  Viruses that infect bacteria
• HIV infection


  1. Attachment to host helper T cells

  2. Injection of RNA and reverse transcriptase

  3. Conversion of RNA to DNA

  4. Integration of viral DNA into host cell nucleus

  5. Production of new virus particles
• HIV positive


  Infected with HIV virus that can develop into AIDS
• AIDS


  Destruction of immune system by replicating HIV viruses in helper T cells
• Influenza


  Infects ciliated cells in gas exchange surfaces, can lead to secondary infections like pneumonia
• Tobacco mosaic virus


  Infects tobacco plants, causes damage to leaves, flowers and fruits
• Protoctista


  Eukaryotic, single-celled or colonial organisms, few are pathogenic but very dangerous
• Malaria


  Caused by Plasmodium protoctista, transmitted by mosquitoes, infects red blood cells and liver
• Potato blight


  Caused by Phytophthora infestans protoctista, damages leaves and fruit
• Fungi


  Eukaryotic, mainly cause plant diseases, release enzymes to digest host tissue
• Black sigatoka


  Caused by Mycosphaerella fijiensis fungus, damages banana leaves
• Ringworm


  Caused by Trichophyton verrucosum fungus, infects skin in circular patterns
• Athlete's foot


  Caused by Tineapedis fungus, infects skin between toes
• Factors affecting pathogen transmission

  • Hot climates
  • Poverty/developing countries
  • Lack of sanitation/fresh water
  • Overcrowding
  • Limited access to medicines and vaccines
• Modes of pathogen transmission

  • Direct: contact, inoculation, ingestion
  • Indirect: vectors, droplets, fomites
• Plant defences


  Physical barriers (bark, cuticles), antibacterial chemicals, repelling insects
• Animal primary defences


  • Skin, blood clotting, mucus, cilia, lysozymes, inflammation, phagocytosis
• Secondary immune response


  Specific response by B cells and T cells to particular antigens
• B cells


  Mature in bone marrow, responsible for humoral immune response
• T cells


  Mature in thymus, responsible for cell-mediated immune response
• Second line of defense

  1. Specific response

  2. Lymphocytes (B cells and T cells) respond to particular shaped antigens

  3. B cells and T cells made in bone marrow, T cells mature in thymus
• Cell-mediated response


  1. T cell receptors bind to antigen on antigen presenting cells

  2. T cells divide rapidly (clonal expansion)

  3. T helper cells activate B cells and macrophages

  4. T memory cells

  5. T killer cells destroy infected/abnormal cells
• Antigen presenting cells


  Cells that present non-self antigens on their surface, e.g. infected body cells, phagocytes, transplant cells, cancer cells
• Humoral response


  1. T helper cells activate B cells

  2. B cells rapidly divide (clonal expansion)

  3. B cells differentiate into plasma cells and memory B cells

  4. Plasma cells produce antibodies
• Antibodies


  Globular quaternary structure proteins with variable regions that bind to specific antigens
• Functions of antibodies


  • Agglutination (clumping pathogens)
  • Opsonisation (marking pathogens for phagocytosis)
  • Antitoxin (binding to toxins)
• Primary immune response


  1. First exposure to pathogen, takes time to produce antibodies, get symptoms

  2. Secondary immune response

  3. Memory B cells rapidly produce antibodies, minimal/no symptoms
• Passive immunity


  Antibodies introduced directly, temporary immunity
• Active immunity


  Exposure to pathogen/antigen, long-term immunity (natural or artificial via vaccination)
• Cell recognition prevents lymphocytes destroying self cells
• Autoimmune disease


  Immune system identifies self cells as foreign and attacks them
• Autoimmune diseases


  • Rheumatoid arthritis
  • Lupus
• Immunization/Vaccination


  Artificial active immunity, triggers primary response with few symptoms, allows rapid secondary response
• Vaccines not always effective long-term due to pathogen mutation and antigen variability
• Epidemic


  Disease spreads rapidly on national level