Infection and Response

• Pathogens

  Microorganisms that cause infectious disease, including viruses, bacteria, protists and fungi
• Viruses

  • Very small
  • Move into cells and use the biochemistry to make many copies of itself
  • This leads to the cell bursting and releasing all of the copies into the bloodstream
  • The damage and destruction of the cells makes the individual feel ill
• Bacteria


  • Small
  • Multiply very quickly through dividing by a process called binary fission
  • Produce toxins that can damage cells
• Protists


  • Some are parasitic, meaning they use humans and animals as their hosts (live on and inside, causing damage)
• Fungi


  • Can be single celled or have a body made of hyphae (thread-like structures)
  • Can produce spores which can be spread to other organisms
• Ways pathogens are spread

  • Direct contact-touching contaminated surfaces
  • By water-drinking or coming into contact with dirty water
  • By air-pathogens can be carried in the air and then breathed in
• Reducing the damage disease causes to populations

  1. Improving hygiene: Hand washing, using disinfectants, isolating raw meat, using tissues and handkerchiefs when sneezing

  2. Reducing contact with infected individuals

  3. Removing vectors: Using pesticides or insecticides and removing their habitat

  4. Vaccination: Injecting a small amount of a harmless pathogen into an individual's body to make them immune
• Vaccination


  Replicates the first infection so that when the person is exposed to the real disease they do not feel any symptoms, just like in a secondary infection
• Components of a vaccine

  • Dead or inactivated form of the pathogen
  • Stimulates white blood cells to produce antibodies complementary to the antigens on the pathogen
• Advantages of vaccination

  • They have eradicated many diseases so far (e.g smallpox) and reduced the occurrence of many (e.g rubella)
  • Epidemics (lots of cases in an area) can be prevented through herd immunity
• Disadvantages of vaccination

  • They are not always effective in providing immunity
  • Bad reactions (such as fevers) can occur in response to vaccines (although very rare)
• Antibiotics

  Medicines that kill bacterial pathogens inside the body, without damaging body cells
• Antibiotics cannot kill viruses as they use body cells to reproduce, meaning any drugs that target them would affect body tissue too</b>
• Painkillers


  Only treat the symptoms of the disease, rather than the cause
• Different antibiotics are effective against different types of bacteria, so receiving the correct one is important</b>
• The great concern is that bacteria are becoming resistant to antibiotics</b>
• How antibiotic resistance develops

  1. Mutations can occur during reproduction resulting in certain bacteria no longer being killed by antibiotics

  2. When these bacteria are exposed to antibiotics, only the non-resistant one die

  3. The resistant bacteria survive and reproduce, meaning the population of resistant bacteria increases
• Preventing the development of antibiotic resistant strains

  1. Stop overusing antibiotics- this unnecessarily exposes bacteria to the antibiotics

  2. Finishing courses of antibiotics to kill all of the bacteria
• Many drugs were initially discovered in plants and microorganisms. New drugs today are mainly synthesised by chemists</b>
• Testing new drugs

  They need to be tested for toxicity, efficacy (how well they carry out their role) and dose, using preclinical testing and clinical trials
• The great concern is that bacteria are becoming resistant to antibiotics
• Development of antibiotic resistance

  1. Mutations occur during reproduction resulting in certain bacteria no longer being killed by antibiotics

  2. When these bacteria are exposed to antibiotics, only the non-resistant ones die

  3. The resistant bacteria survive and reproduce, meaning the population of resistant bacteria increases

  4. This means that antibiotics that were previously effective no longer work
• Ways to prevent development of resistant bacteria strains

  • Stop overusing antibiotics - this unnecessarily exposes bacteria to the antibiotics
  • Finishing courses of antibiotics to kill all of the bacteria
• Discovery and Development of Drugs

  Many drugs were initially discovered in plants and microorganisms. New drugs today are mainly synthesised by chemists. They need to be tested for toxicity, efficacy (how well they carry out their role) and dose, using preclinical testing and clinical trials.
• Drugs discovered in plants

  • Aspirin (originates from willow)
  • Digitalis (originates from foxgloves)
• Penicillin


  Alexander Fleming was growing bacteria on plates and found mould (Penicillium mould) on his culture plates, with clear rings around the mould indicating there was no longer any bacteria there. He found that the mould was producing a substance called penicillin, which killed bacteria.
• Drug testing

  1. Preclinical testing: using cells, tissues and live animals

  2. Clinical testing: using volunteers and patients

  3. First tested on healthy volunteers with a low dose to ensure there are no harmful side effects

  4. Then tested on patients to find the most effective dose

  5. Patients split into two groups with one group receiving the drug and one receiving a placebo to observe the effect

  6. Results peer reviewed by other scientists to check for repeatability
• Monoclonal antibodies


  Identical antibodies, that have been produced from the same immune cell. As a result of their ability to bind to only one protein antigen, they can be used to target chemicals and cells in the body and so have many different medical uses.
• Production of monoclonal antibodies

  1. Scientists obtain mice lymphocytes (a type of white blood cell that make antibodies but cannot divide), which have been stimulated to produce a specific antibody

  2. They are combined with tumour cells (do not make antibodies but divide rapidly), to form a cell called a hybridoma

  3. The hybridoma can divide to produce clones of itself, which all produce the same antibody

  4. The antibodies are collected and purified
• Uses of monoclonal antibodies

  • Pregnancy tests
  • In laboratories to measure and monitor levels of hormones or chemicals
  • In research to find or identify certain molecules on a cell or tissue
  • In the treatment of disease, e.g. cancer
• Advantages of using monoclonal antibodies

  • They only bind to specific cells, meaning healthy cells are not affected
  • They can be engineered to treat many different conditions
  • We are now able to produce mouse-human hybrid cells to reduce the chance of triggering an immune response
• Disadvantages of using monoclonal antibodies

  • It is difficult to attach monoclonal antibodies to drugs
  • They are expensive to develop
  • As they were produced from mice lymphocytes, they often triggered an immune response when used in humans
• Plants can also be affected by viral, bacterial and fungal pathogens
• Common signs of plant diseases

  • Stunted growth
  • Spots on leaves
  • Areas of decay
  • Abnormal growths
  • Discolouration
  • Pests on leaves
• Identifying plant diseases

  1. Using a gardening manual or website

  2. Identifying the pathogen by observing the infected plant in a laboratory

  3. Using monoclonal antibodies in testing to identify the pathogen
• Plant diseases to be learnt

  • Tobacco mosaic virus (viral disease)
  • Black spot (fungal disease)
  • Aphids (insect pest)
• Nitrate deficiency

  Can stunt plant growth as nitrates in the soil convert sugars made in photosynthesis into proteins needed for growth
• Magnesium deficiency

  Can cause chlorosis as magnesium is needed to make chlorophyll, the green pigment vital for photosynthesis
• Plant defence responses

  • Physical defences: Tough waxy cuticle, cellulose cell walls, layers of dead cells around stems
  • Chemical defences: Poisons, antibacterial compounds
  • Mechanical defences: Thorns and hairs, leaf movements, mimicry