Biology Module 4.1

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


  Classified by shape or cell wall
• Shapes of prokaryotic cells

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

  Determined by cell wall type
• Tuberculosis


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


  Caused by Streptococcus pneumoniae or Neisseria meningitidis, infects the brain's meninges, very serious and can be fatal, 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, envelope, RNA core and reverse transcriptase enzyme, attaches to and infects helper T cells, can lead to AIDS by destroying the immune system
• 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 immunocompromised
• 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 colonial, few are pathogenic but those that are cause severe symptoms
• 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, release enzymes to digest host tissue and feed on it
• Black Sigatoka


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


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


  Caused by Tineapedis fungus, infects human feet, thrives in warm damp areas, causes cracked, scaly and itchy skin, can be treated with antifungal creams
• Factors affecting pathogen transmission

  • Climate
  • Social factors (poverty, sanitation, overcrowding, access to medicine)
• Modes of pathogen transmission

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


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


  • Skin
  • Blood clotting
  • Mucus membranes
  • Lysozymes
  • Sneezing, coughing, vomiting
  • Inflammation
• Phagocytosis


  Phagocytes (macrophages, neutrophils) engulf and digest pathogens, non-specific response
• Stages of phagocytosis


  • Pathogen/damaged cell release chemicals
  • Phagocytes attracted
  • Phagocyte receptors bind to pathogen
  • Phagocyte engulfs pathogen
  • Phagosome and lysosome fuse to digest pathogen
  • Phagocyte presents pathogen antigen
• Adaptive immune system components

  • B cells
  • T cells
• 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 antigen receptor

  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


  Involves antibodies produced by B cells
• Antibody


  Globular quaternary structure protein with variable region that binds to antigen and constant region
• Antibody functions


  1. Agglutination - clumping of pathogens

  2. Opsonisation - marking pathogens for phagocytosis

  3. Antitoxin - binding to and neutralising toxins
• Humoral response process


  1. Activated T helper cells bind to B cells

  2. B cells undergo clonal selection and expansion

  3. B cells differentiate into plasma cells that produce antibodies or memory B cells
• Primary immune response


  First exposure to pathogen, takes time to produce antibodies