MODULE 4 VIDEO

• Communicable diseases are caused by pathogens and that includes bacteria, viruses, protoctista and fungi
• Pathogens cause harm through directly damaging the tissue or through the release of toxins
• Examples of diseases caused by different pathogens

  • Bacterial diseases
  • Viral diseases
  • Protoctista diseases
  • Fungal diseases
• Prokaryotic cells

  Classified by their shape or cell wall
• Shapes of prokaryotic cells

  • Rods
  • Spherical
  • Comma
  • Spiraled
  • Corkscrew
• Gram-positive and gram-negative bacteria


  Determined by the two types of cell walls
• Tuberculosis

  Caused by Mycobacterium tuberculosis or Mycobacterium bovis, damages lung tissue and suppresses immune system, can be cured with antibiotics and prevented through vaccination
• Bacterial meningitis

  Caused by Streptococcus pneumoniae or Neisseria meningitidis, infects the brain's meninges, very serious and spreads quickly, can be cured with antibiotics and some forms can be vaccinated against
• Ring rot

  Caused by Clavibacter michiganensis, infects potatoes, tomatoes and aubergines, damages leaves, tubers and fruit
• 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

  Consists of capsid, genetic material (RNA), reverse transcriptase enzyme, envelope and attachment proteins, infects and destroys helper T cells, can lead to AIDS
• Influenza

  Caused by Orthomyxoviridae, infects ciliated cells in gas exchange surfaces, can result in harmful secondary infections like pneumonia, more severe in young, elderly and those with weakened immune systems
• Tobacco mosaic virus

  Infects tobacco plants, causes damage to leaves resulting in mosaic pattern, affects photosynthesis, no cure but resistant plant strains have been developed
• Protoctista

  Eukaryotic, single-celled or in colonies, few are pathogenic but extremely dangerous
• Malaria

  Caused by Plasmodium protoctista, transmitted by mosquitoes, infects red blood cells and liver, no vaccine or cure but preventative medicines exist
• Potato blight

  Caused by Phytophthora infestans protoctista, damages leaves and fruit, no cure but resistant plant strains have been developed
• Fungi

  Eukaryotic, mainly cause plant diseases, can be multicellular or single-celled, release enzymes to digest host tissue
• Black Sigatoka

  Caused by Mycosphaerella fijiensis fungus, infects bananas, damages leaves preventing photosynthesis, can be treated with fungicides and resistant strains have been developed
• Ringworm

  Caused by Trichophyton verrucosum fungus, infects many mammals including humans, causes white crusty circles on skin, not harmful but can cause itching
• Athlete's foot

  Caused by Tineapedis fungus, infects humans, thrives in warm damp regions like between toes, causes cracked, scaly and itchy skin, can be cured with antifungal creams
• Factors that can increase pathogen transmission

  • Hot climates
  • Poverty/developing countries
  • Lack of sanitation infrastructure
  • Lack of fresh water
  • Lack of medicines/vaccines
  • Overcrowding
• Modes of pathogen transmission

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

  Physical barriers like bark and cuticles, antibacterial chemicals and proteins, ability to repel insects
• Animal primary defences

  Skin, blood clotting, mucus membranes, lysozymes, sneezing/coughing/vomiting, inflammation
• Phagocytosis

  Phagocytes like macrophages and neutrophils engulf and digest pathogens, non-specific response
• Adaptive immune response

  Involves B cells and T cells, specific response 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. Pathogen stopped from getting in or destroyed

  2. Specific response to particular shaped antigens

  3. Involves B lymphocytes and T lymphocytes

  4. B cells mature in bone marrow

  5. T cells mature in thymus
• T cells

  Responsible for cell-mediated response
• Cell-mediated response

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

  2. T cells divide rapidly by mitosis (clonal expansion)

  3. T helper cells activate B cells and macrophages

  4. T memory cells retain receptor for future infection

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

  Cells that present non-self antigen on their surface, e.g. infected body cells, phagocytes that have engulfed pathogens, transplanted organ cells, cancer cells
• T killer cell action

  1. Release perforin protein

  2. Perforin embeds in cell membrane and creates pores

  3. Causes cell contents to leak out or water to enter, leading to cell death
• Humoral response

  1. T helper cells stimulate 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 protein with variable region that binds to antigen
• Antibody functions

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

  1. First exposure to pathogen

  2. Takes days to produce enough antibodies

  3. Causes symptoms as pathogen causes damage
• Secondary immune response

  1. Re-exposure to same pathogen

  2. Memory B cells rapidly produce antibodies

  3. Pathogen destroyed before symptoms appear
• Active immunity

  Immunity gained from being exposed to pathogen or antigen, either naturally or through vaccination