Communicable diseases:

• Pathogens


  • Viruses take over the cell metabolism. The viral genetic material gets into the host cell and is inserted into the host DNA. The virus then uses the host cell to make new viruses which then bursts out the cell destroying it and then spread to infect other cells
  • Some protocista takes over cells and break them open as the new generation emerge but dont take over the genetic material of the cell. They digest and use the cell contents as they reproduce
  • Fungi digest living cells and destroy them. This combined with the response if the body to the damage gives the symptoms of disease
• Pathogen:


  • Most bacteria produce toxins that poison or damage the host cells causing disease. Some bacterial toxins damage the host cells by breaking down the cell membranes, some toxins damage or inactivate enzymes and some interfere with host cells genetic material so cells cannot divide. These toxins are a by-product of the normal functioning of the bacteria
• Plant diseases:


  Ring rot: bacterial disease of potatoes caused by gram-positive bacterium. Damages leaves and fruit. Destroys up to 80% of the crop and is no cure. Once it infects a field it cannot be used for 2 years

  TMV: Virus infects tobacco plants. Damages leaves and fruit stunting growth and reducing yields. Resistant crop strains are available but no cure

  Potato blight: Fungus, the hyphae penetrates host cells, destroying leaves. No cure but resistant strains, careful management and chemical treatments can reduce infection risk

  Black Sigatoka: banana disease attacks and destroys leafs
• Animal disease:


  TB: Bacteria, destroys and damages lung tissue suppressing immune system so body is less likely to fight off other disease. TB is curable by antibiotics and preventable by vaccinations

  Bacterial meningitis: Bacteria of the brain which can spread into the rest of the body causing blood poisoning and rapid death. Mainly affects young children and teenagers. A blotchy red/purple rash that doesn't disappear. Antibiotics can cure it if given early and vaccinations
• Animal disease:


  HIV/AIDS: Virus targets T helper cells in the immune system. Destroys immune system so are open to other disease such as cancer, HIV contains an enzyme that reverse transcriptase. Passed by bodily fluids. No vaccine or cure

  Influenza: virus affects the ciliated epithelial cells in the gas exchange system, kills them leaves the airways open to secondary infections. Can be fatal. Vaccine no cure

  Malaria: Protocista spread by vectors. Produces inside the female mosquito when she feeds it leads to the disease. Invades red blood cells No vaccine limited cures
• Animal disease:


  Ring worm: fungal causes grey-white crusty infectious circular areas of skin. Its not damaging but can be itchy Anti-fungal cream is a cure

  Athletes foot: Digests warm moist skin between toes. Causes cracking and scaling which is itchy and becomes sore. Anti-fungal cream is a cure
• Plants defence against pathogen:

  Physical defences:

  • Produce callous deposited between the cell walls and cell membrane in cells next to the infected ones. This acts as a barrier preventing the pathogen from entering the plant cells around the site of infection
  • Lignin is added making the mechanical barrier to invasion stronger and thicker
  • Callos blocks sieve plates in the phloem and sealing them off the infected part and preventing spread, deposited in the plasmodesmata between infected cells sealing them off from the healthy cells
• Plant defence against pathogen:

  Chemical defences:

  • insect repellents: pine resin
  • Insecticides: caffeine toxic to fungi and insects
  • Antibacterial compounds: phenol
  • Antifungal compounds: Phenol
• Non-specfic animal defences:


  Skin: prevents entry of pathogens has skin flora healthy microorganisms that outcompete pathogens for space on body surface. Produces sebum oily substance inhibits growth of pathogens

  Airways lines with mucous membrane: Secrete sticky mucus traps microorganisms that contain lysosomes which destroy bacterial cell wall
• Non-specific animal defences:

  Blood clotting and wound repair:

  • Thromboplastin an enzyme that triggers a cascade reaction resulting in the formation of a blood clot
  • Serotonin makes smooth muscle in the walls of blood vessels contract so they narrow and reduce the supply of blood to the area
• Non-specific animal defences:

  Inflammatory response:

  • mast cells activate in damaged tissue and release histamines and cytokines
  • Histamines make blood vessels more leaky so blood plasma is formed out known as tissue fluid. Tissue fluid causes swelling and pain
  • Cytokines attract white blood cells to the site, they dispose pathogen by phagocytosis
• Phagocytosis:

  1. Pathogens produce chemicals that attract phagocytes
  2. Phagocytes recognise pathogen as non-self and binds to it
  3. The phagocyte engulfs the pathogen and encloses it in a vesicle called a phagosome
  4. Phagosome combines with lysosome to form a phagolysosome
  5. Enzymes from the lysosome digest and destroy the pathogen
• Phagocytosis:

  When a macrophage has digested the pathogen, it combines with the antigen from the pathogen surface membrane with special glycoproteins in the cytoplasm called major histocompatiablity complex (MHC)

  The MHC complex moves these pathogen antigens to the macrophages own surface membrane becoming an antigen-presenting cell (APC). These antigens now stimulate other cells involved in the specific immune system response
• Helping chemicals:


  Phagocytes that have engulfed a pathogen produce a chemical called cytokines which act as cell-signalling molecules, informing other phagocytes that the body is under attack stimulating them to move to the site of infection. Can also increase the body temp

  Opsonins chemicals that bind to pathogens and tag them so they can be more easily identified by phagocytes.

  Phagocytes have receptors on their cell membrane that bind to common opsonins and the phagocyte engulfs the pathogen
• How antibodies defend the body:

  1. The antibody of the antigen-antibody complex act as opsonins so the complex is easily engulfed and digested by phagocytes
  2. Most pathogens can no longer effectively invade the host cell once they are part of an antigen-antibody complex
  3. Antibodies act as agglutinins causing pathogens carrying antigen-antibody complex to clump together. This helps prevent them from spreading through the body and makes it easier for phagocytes to engulf pathogens at the same time
  4. Antibodies can act as anti-toxins, binding to the toxin produced by pathogens making them harmless