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
  • They 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)
  • They can produce spores which can be spread to other organisms
• Ways pathogens are spread

  • Direct contact (touching contaminated surfaces, kissing, contact with bodily fluids, microorganisms from faeces, infected plant material)
  • By water (drinking or coming into contact with dirty water)
  • By air (pathogens can be carried in the air and then breathed in, droplet infection)
• Ways to reduce the damage of disease

  • Improving hygiene (hand washing, using disinfectants, isolating raw meat, using tissues and handkerchiefs when sneezing)
  • Reducing contact with infected individuals
  • Removing vectors (using pesticides or insecticides and removing their habitat)
  • Vaccination
• Vaccination


  Injecting a small amount of a harmless pathogen into an individual's body, so they can become immune to it and not get infected
• Herd immunity


  When a large proportion of the population is vaccinated, the spread of the pathogen is reduced as there are less people to catch the disease from
• Measles


  • Symptoms: Fever and red skin rash, can lead to other problems such as pneumonia, encephalitis and blindness
  • Spread by droplet infection
  • Prevented by vaccinations for young children
• HIV


  • Symptoms: Initially flu-like symptoms, then the virus attacks the immune system and leads to AIDS
  • Spread by sexual contact or exchange of bodily fluids
  • Prevented by using condoms, not sharing needles, screening blood, and mothers with HIV bottle-feeding their children instead of breastfeeding
  • Treated with antiretroviral drugs
• Tobacco mosaic virus


  • Symptoms: Discolouration of the leaves, the affected part of the leaf cannot photosynthesise resulting in the reduction of the yield
  • Spread by contact between diseased plants and healthy plants, insects act as vectors
  • Prevented by good field hygiene and pest control, growing TMV-resistant strains
• Salmonella food poisoning


  • Symptoms: Fever, stomach cramps, vomiting, diarrhoea
  • Spread through raw meat and eggs, unhygienic conditions
  • Prevented by vaccinating poultry, keeping raw meat away from cooked food, avoiding washing it, washing hands and surfaces when handling it, cooking food thoroughly
• Gonorrhoea


  • Symptoms: Thick yellow or green discharge from the vagina or penis, pain when urinating
  • Spread through unprotected sexual contact
  • Prevented by using contraception such as condoms and antibiotics
• Rose black spot


  • Symptoms: Purple or black spots on leaves of rose plants, reduces the area of the leaf available for photosynthesis, leaves turn yellow and drop early
  • Spread by the spores of the fungus in water or by wind
  • Prevented by using fungicides or stripping the plant of affected leaves (have to be burnt)
• Malaria


  • Symptoms: Fevers and shaking
  • Spread by the female Anopheles mosquito, in which the protists reproduce sexually
  • Prevented by using insecticide coated insect nets while sleeping, removing stagnant water to prevent the vectors from breeding, travellers taking antimalarial drugs to kill parasites that enter the blood
• Non-specific defence system


  • The skin (acts as a physical barrier, produces antimicrobial secretions, has good microorganisms known as skin flora)
  • The nose (has hairs and mucus which prevent particles from entering the lungs)
  • The trachea and bronchi (secrete mucus to trap pathogens, have cilia to waft mucus upwards)
  • The stomach (produces hydrochloric acid that kills pathogens)
• Specific immune system


  • Phagocytosis (white blood cells engulf and consume pathogens)
  • Producing antibodies (each pathogen has an antigen that a specific complementary antibody can bind to)
  • Producing antitoxins (neutralise toxins released by pathogens)
• Antibiotics


  Medicines that kill bacterial pathogens inside the body, without damaging body cells. They cannot kill viruses as they use body cells to reproduce.
• Painkillers


  Only treat the symptoms of the disease, rather than the cause.
• The great concern is that bacteria are becoming resistant to antibiotics
• Ways to prevent the development of resistant bacterial strains

  • Stop overusing antibiotics (this unnecessarily exposes bacteria to the antibiotics)
  • Finishing courses of antibiotics to kill all of the bacteria
• Many drugs were initially discovered in natural sources
• Epidemics (lots of cases in an area) can be prevented through herd immunity
• Bad reactions (such as fevers) can occur in response to vaccines (although very rare)
• Mutations can occur during reproduction resulting in certain bacteria no longer being killed by antibiotics


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

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

  3. This means that antibiotics that were previously effective no longer work
• Preclinical testing


  Using cells, tissues and live animals
• Clinical testing


  Using volunteers and patients
• Clinical testing


  1. It is first tested on healthy volunteers with a low dose to ensure there are no harmful side effects

  2. The drugs are then tested on patients to find the most effective dose

  3. To test how well it works, patients are split into two groups with one group receiving the drug and one receiving a placebo (appears to look like the drug but has no active ingredient so no effect) so the effect of the new drug can be observed
• Single-blind


  Only the doctor knows whether the patient is receiving the drug
• Double blind


  Neither the patient or doctor knows whether they are receiving the drug, removing any biases the doctor may have when they are recording the results
• The results then need to be peer reviewed by other scientists to check for repeatability
• 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
• Common signs of plant diseases

  • Stunted growth: indicating nitrate deficiency
  • Spots on leaves: indicating black spot fungus on roses
  • Areas of decay: black spot fungus on roses, blights on potatoes
  • Abnormal growths: crown galls caused by bacterial infection
  • Malformed stems or leaves: due to aphid infestation
  • Discolouration: indicating magnesium deficiency, or tobacco mosaic virus
  • Pests on leaves: such as caterpillars
• Plant diseases that need to be learnt

  • Tobacco mosaic virus (viral disease)
  • Black spot (fungal disease)
  • Aphids (as insects)
• Nitrate deficiency


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


  Can cause chlorosis as magnesium is needed to make chlorophyll, the green pigment vital in photosynthesis
• Physical defences of plants

  • Tough waxy cuticle stops entry into leaves
  • Cellulose cell walls form a physical barrier into the cells
  • Plants have layers of dead cells around stems (such as bark) which stop pathogens entering. The dead cells fall off with the pathogens
• Chemical defences of plants

  • Poisons (e.g from foxgloves, tobacco plants, deadly nightshades, yew) deter herbivores (organisms that eat plants)
  • Antibacterial compounds kill bacteria, such as mint plant and witch hazel