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 (a common example is the droplet infection, which is when sneezing, coughing or talking expels pathogens in droplets which can be breathed in)
• Ways to reduce the damage of disease to populations

  • 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: By injecting a small amount of a harmless pathogen into an individual's body, they can become immune to it so it will not infect them. This means they cannot pass it on.
• Measles


  • Symptoms: Fever and red skin rash, can lead to other problems such as pneumonia (lung infection), encephalitis (brain infection) and blindness
  • How it is spread: Droplet infection
  • How it is being prevented: Vaccinations for young children to reduce transmission
• HIV


  • Symptoms: Initially flu-like symptoms, then the virus attacks the immune system and leads to AIDS (a state in which the body is susceptible to many different diseases)
  • How it is spread: By sexual contact or exchange of bodily fluids such as blood
  • How it is being prevented: The spread - Using condoms, not sharing needles, screening blood when it is used in transfusions, mothers with HIV bottle-feeding their children instead of breastfeeding
  • The development to AIDS - Use of antiretroviral drugs (stop the virus replicating in the body)
• Tobacco mosaic virus


  • Symptoms: Discolouration of the leaves, the affected part of the leaf cannot photosynthesise resulting in the reduction of the yield
  • How it is spread: Contact between diseased plants and healthy plants, insects act as vectors
  • How it is being prevented: Good field hygiene and pest control, growing TMV-resistant strains
• Salmonella food poisoning


  • Symptoms: Fever, stomach cramps, vomiting, diarrhoea (all caused by the toxins they secrete)
  • How it is spread: These bacteria can be found in raw meat and eggs, unhygienic conditions
  • How it is being prevented: Poultry are vaccinated against Salmonella, keeping raw meat away from cooked food, avoid washing it, wash hands and surfaces when handling it, cook food thoroughly
• Gonorrhoea


  • Symptoms: Thick yellow or green discharge from the vagina or penis, pain when urinating
  • How it is spread: It is a sexually transmitted disease spread through unprotected sexual contact
  • How it is being prevented: By using contraception such as condoms and antibiotics (used to be treated with penicillin but many resistant strains are developing)
• 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
  • How it is spread: The spores of the fungus are spread in water (rain) of by wind
  • How it is being prevented: By using fungicides or stripping the plant of affected leaves (have to be burnt)
• Malaria


  • Symptoms: Fevers and shaking (when the protists burst out of blood cells)
  • How it is spread: The vector is the female Anopheles mosquito, in which the protists reproduce sexually. When the mosquito punctures the skin to feed on blood, the protists enter the human bloodstream via their saliva.
  • How it is being prevented: 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 your lungs
  • The trachea and bronchi - secrete mucus to trap pathogens, have cilia to waft mucus upwards
  • The stomach - produces hydrochloric acid that kills any pathogens in your mucus, or food and drink
• Specific immune system


  • Phagocytosis - white blood cells engulf and consume pathogens
  • Producing antibodies - each pathogen has an antigen which a specific complementary antibody can bind to
  • Producing antitoxins - they neutralise the toxins released by the pathogens
• Vaccination


  • Involves making an individual immune to a certain disease by injecting a small amount of a harmless pathogen into their body
  • This stimulates white blood cells to produce antibodies complementary to the antigens on the pathogen
  • When the person is exposed to the real disease, the antibodies can be produced much quicker so the pathogen can be destroyed and the symptoms are not felt
• Antibiotics


  • Medicines that kill bacterial pathogens inside the body, without damaging body cells
  • Different antibiotics are effective against different types of bacteria, so receiving the correct one is important
  • Their use has decreased the number of deaths from bacterial diseases
  • The great concern is that bacteria are becoming resistant to antibiotics due to mutations
• Painkillers (such as aspirin) only treat the symptoms of the disease, rather than the cause
• Many drugs were initially discovered in natural sources such as plants and microorganisms
• 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)
• The great concern is that bacteria are becoming resistant to antibiotics
• 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
• To prevent the development of these resistant strains

  • Stop overusing antibiotics- this unnecessarily exposes bacteria to the antibiotics
  • Finishing courses of antibiotics to kill all of the bacteria
• Toxicity


  How poisonous a drug is
• Efficacy


  How well a drug carries out its role
• Preclinical testing


  Using cells, tissues and live animals to test new drugs
• Clinical trials


  Using volunteers and patients to test new drugs
• New drugs today are mainly synthesised by chemists
• Plants


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


  1. Alexander Fleming was growing bacteria on plates

  2. He found mould (Penicillium mould) on his culture plates, with clear rings around the mould indicating there was no longer any bacteria there

  3. He found that the mould was producing a substance called penicillin, which killed bacteria
• Placebo


  Appears to look like the drug but has no active ingredient so no effect
• 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
• Monoclonal antibodies


  Identical antibodies, that have been produced from the same immune cell
• Producing 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
  • In research to find or identify certain molecules on a cell or tissue
  • In the treatment of disease, e.g. cancer
• Tumour markers


  Antigens on cancer cell membranes that are not found on normal body cells