Disease, defence and immunity

• Four groups of pathogens that can cause communicable diseases are:

  • Bacteria
  • Fungi
  • Protoctista
  • Viruses
• Mycobacterium tuberculosis causes disease by:

  • Triggering an inflammatory response by infecting phagocytes in the lungs
  • Sealing infected phagocytes in waxy-coated tubercles to remain dormant initially
  • Becoming active and destroying lung tissue if the immune system weakens
• HIV results in the symptoms of AIDS by:

  • Binding to receptors on T Helper cells
  • Replicating inside T Helper cells, killing or damaging them
  • Developing AIDS when there are too few T Helper cells for the immune system to function properly
  • Individuals having compromised immune systems, making them vulnerable to other infections
• The tobacco mosaic virus causes disease in plants by:

  • Affecting plants and being mainly transmitted via infected sap
  • Containing ssRNA, which is directly transcribed by host cells to assemble new virus cells
  • Entering other cells via plasmodesmata and then entering xylem and phloem, causing stunted growth and mottled leaves
• Malaria is caused by:

  • Female Anopheles mosquitoes acting as vectors for Plasmodium spp. protoctista when transferring saliva during feeding
  • The parasite reproducing asexually in red blood cells in the liver, causing lysis
• Potato/tomato late blight is caused by the protoctista Phytophthora infestans, mainly transmitted via spores. Ring rot of potatoes is caused by the bacterium Clavibacter michiganensis subspecies Sepedonicus, mainly transmitted by planting infected seeds or contact with fomites. Plant-to-plant transmission is rare
• Three fungal infections are:

  • Mycosphaerella fijiensis causing black sigatoka in banana plants
  • Athlete’s foot in humans (tinea pedis) caused by a range of fungi that can also affect hands or nails
• Communicable pathogens are transmitted directly through:

  • Inhalation (droplet infection)
  • Skin-to-skin contact or exchange of fluids
  • Active penetration of skin using enzymes or passive entry through wounds, hair follicles, or sweat glands
• Communicable pathogens are transmitted indirectly through:

  • Consumption of contaminated food and drink
  • Via a vector, e.g., mosquitoes transmitting Plasmodium parasite
  • Spores
• Living conditions affect disease transmission by:

  • Overcrowding increasing direct transmission
  • Climate determining which organisms can survive, influencing disease prevalence
  • Social factors affecting treatment speed, impacting transmission rates
• Four physical barriers to pathogen entry in plants are:

  • Cellulose cell walls
  • Lignified layer
  • Waxy upper cuticle
  • Old vascular tissue blocked to prevent pathogen spread inside the plant
• Two mechanical responses to infection in plants are:

  • Guard cells closing stomata
  • Production of thick polysaccharide callose to limit pathogen spread
• Necrosis in plants is when injury activates intracellular enzymes that kill cells near the infection site to prevent pathogen spread. Necrosis of woody tissue is known as canker
• Chemical defenses plants use against pathogens include:

  • Terpenoids (essential oils) acting as antibacterials
  • Phenols inhibiting insects by interfering with digestion
  • Alkaloids deterring herbivores by tasting bitter
  • Defensins inhibiting transport channels
  • Hydrolytic enzymes breaking down cell walls of invading organisms
• Five barriers to infection in animals are:

  • Tough keratin layer of skin
  • Blood clotting preventing pathogen entry through skin lesions
  • Hydrochloric acid in the stomach killing bacteria
  • Harmless bacteria in the gut and on the skin surface competing with pathogens
  • Mucous membranes trapping pathogens and secreting antimicrobial enzymes
• Expulsive reflexes in animals are the body's attempts to force foreign substances out, such as sneezing due to irritation of mucous membranes in nostrils and coughing due to irritation of ciliated epithelium in the respiratory tract
• Nonspecific immune system responses to infection include:

  • Inflammation
  • Phagocytosis
  • Digestive action of lysozymes
  • Production of interferon (antiviral agent)
• The process of inflammation involves:

  • Damaged vessels releasing histamines, causing vasodilation
  • Increased blood flow and permeability of blood vessels
  • White blood cells and plasma moving into the infected tissue
• Blood clotting occurs by:

  • Blood platelets forming a plug and releasing chemicals that enhance clotting
  • Prothrombin changing into thrombin, leading to fibrinogen changing into insoluble fibrin that covers the wound
• The two types of white blood cells involved in phagocytosis are neutrophils and macrophages, which can become antigen-presenting cells
• Phagocytosis destroys pathogens by:

  • Phagocyte moving towards the pathogen marked by opsonins via chemotaxis
  • Engulfing the pathogen
• Phagocytosis process to destroy pathogens:

  • Phagocyte moves towards pathogen marked by opsonins via chemotaxis
  • Phagocyte engulfs pathogen via endocytosis to form a phagosome
  • Phagosome fuses with lysosome (phagolysosome)
  • Lysozymes digest pathogen
  • Phagocyte absorbs products from pathogen hydrolysis
• Role of antigen-presenting cells (APCs):

  • Macrophage displays antigen from pathogen on its surface after hydrolysis in phagocytosis
  • Enhances recognition by T Helper cells
  • Secrete cytokines that stimulate specific immune response
• Lysozymes:

  • Digestive enzymes
  • Found in lysosomes and secretions like tears & mucus
  • Damage bacterial cell walls, causing osmotic lysis
• Preparing blood for microscope observation:

  • Smear a drop of blood onto a slide using a spreader held at 45°
  • Add Leishman stain then a buffer and rinse
• Two types of specific immune response:

  • Cell-mediated
  • Humoral
• Cell-mediated response process:

  • T Helper lymphocytes bind to foreign antigen on APC
  • Cell signalling stimulates clonal expansion of T Helper cells and cytotoxic T cells
• Humoral response process:

  • TH lymphocytes bind to foreign antigen on antigen-presenting T cells
  • Release cytokines that stimulate clonal expansion of complementary B lymphocytes
  • B cells differentiate into plasma cells that secrete antibodies
• Structure and function of B and T lymphocytes:

  • Many specific receptors & immunoglobulins on surface
  • B cells differentiate into plasma cells to secrete antibodies
  • 3 types of T cells: T Helper (secrete cytokines), T killer (secrete perforin), T regulator (suppress other immune cells)
• Antibody structure and function:

  • Proteins secreted by plasma cells
  • Quaternary structure: 2 'light chains' held by disulfide bridges, 2 longer 'heavy chains'
  • Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen
• Antibodies lead to pathogen destruction by:

  • Agglutinins form antigen-antibody complexes to enhance phagocytosis
  • Activation of complement
  • Opsonins mark microbes for phagocytes
  • Antitoxins make toxins insoluble via precipitation/neutralisation
• Memory cells:

  • Specialised TH/B cells produced from primary immune response
  • Remain in low levels in the blood
  • Can divide rapidly by mitosis if organism encounters the same pathogen again
• Contrast primary and secondary immune response:

  • Secondary response has faster antibody production, shorter time lag, higher antibody concentration, and remains higher after response
• Compare and contrast passive and active immunity:

  • Passive and active both involve antibodies and can be natural or artificial
  • Passive has no memory cells and antibodies are not replaced, while active produces memory cells
• Examples of passive and active immunity:

  • Passive natural: antibodies in breast milk/placenta, passive artificial: anti-venom
  • Active natural: humoral response to infection, active artificial: vaccination
• Autoimmune disease definition and examples:

  • Immune system produces antibodies against its own tissues
  • Examples: Rheumatoid arthritis, Lupus
• Principles of vaccination:

  • Vaccine contains dead/inactive form of pathogen or antigen
  • Triggers primary immune response and produces memory cells for rapid secondary response
• Endemic vs. epidemic:

  • Endemic: disease occurs routinely in a geographical area
  • Epidemic: temporary rapid increase in disease incidence in a geographical area
• Role of vaccines in preventing epidemics:

  • Routine vaccination reduces carriers of pathogen, resulting in herd immunity
  • Vaccinating close contacts limits spread of pathogen
• Possible natural sources of medicines:

  • Microorganisms like streptomycin, neomycin, fungi like penicillin, plants like Taxol, quinine, digoxin