B3: Infection and Response

• There Are Several Types of Pathogen, pathogens are microorganisms that cause disease
• Types of Pathogen

  • Virus
  • Bacteria
  • Protists
  • Fungi
• Bacteria


  • Very small living cells
  • Produce rapidly inside your body
  • Can make toxins that damage the body
• Viruses

  • Not cells, much smaller than bacteria
  • Live inside your cells and replicate themselves
  • Cause cell damage when the cell bursts
• Protists

  • Single-celled eukaryotes
  • Many different types
  • Some are parasites that live in or on other organisms
• Fungi

  • Some are single-celled, others have a body made up of hyphae (thread-like structures)
  • Hyphae can grow and penetrate human skin and plant surfaces, causing disease
  • Produce spores that can spread to other plants and animals
• Ways Pathogens Can Be Spread

  • Water
  • Air
  • Direct contact
• How viruses cause cell damage

  1. Virus enters cell

  2. Virus takes control of cell machinery

  3. Cell bursts, releasing new viruses
• Viral Diseases

  • Measles
  • HIV
  • Tobacco mosaic virus
• Fungal Disease


  • Rose black spot
• Protist Disease

  • Malaria
• Bacterial Diseases


  • Salmonella food poisoning
  • Gonorrhoea
• Ways to Reduce/Prevent Disease Spread

  • Being hygienic
  • Destroying vectors
  • Isolating infected individuals
  • Vaccination
• Human Body's Defence System

  • Skin as barrier
  • Mucus and cilia trapping particles
  • Stomach acid killing pathogens
  • White blood cells attacking pathogens
• How Immune System Attacks Pathogens

  1. Phagocytosis (consuming pathogens)

  2. Producing antibodies

  3. Producing antitoxins
• Vaccination

  Injecting dead or weakened pathogens to stimulate immune system to produce antibodies
• Pros of Vaccination

  • Control of many communicable diseases
  • Prevention of disease outbreaks
• Cons of Vaccination

  • Vaccines don't always work
  • Potential for bad reactions
• Drugs

  • Some relieve symptoms, others cure the underlying problem
  • Antibiotics kill or prevent growth of bacteria, but don't work on viruses
  • Overuse of antibiotics can lead to bacterial resistance
• Drugs do a certain kind of thing - reduce the thing which is the underlying cause
• Antibiotics

  Work differently - they kill (or prevent the growth of) the bacteria causing the problem without harming your own body
• Different antibiotics target different types of bacteria, so it's important to be treated with the right one
• Antibiotics don't destroy viruses (a viral disease)
• Viruses reproduce inside cells, which makes it very difficult to develop drugs that destroy just the virus without killing the body's cells
• The use of antibiotics has greatly reduced the number of deaths from communicable diseases caused by bacteria
• How bacteria become resistant to antibiotics

  1. Bacteria can mutate

  2. Mutations cause them to be resistant (not killed by) antibiotics

  3. If you have an infection, some of the bacteria might be resistant to antibiotics

  4. The resistant bacteria will survive and reproduce, increasing the population of the resistant strain

  5. This is an example of natural selection
• To slow down the development of resistant strains

  • Doctors should be careful prescribing antibiotics, only for serious infections not minor ones
  • Patients should finish the whole course of antibiotics and not stop once they feel better
• Many drugs originally came from plants
• Plants produce chemicals to defend themselves against pests and pathogens


  Some of these chemicals can be used as drugs to treat human diseases or relieve symptoms
• Drugs developed from plant chemicals

  • Aspirin from willow
  • Digitalis from foxgloves
• Nowadays, drugs are made on a large scale in the pharmaceutical industry, synthesised by chemists in labs
• Before drugs can be given to the general public, they have to go through extensive testing procedures
• The three main stages in drug testing

  1. Preclinical testing on human cells and tissues in the lab

  2. Testing on live animals to test efficacy, toxicity and dosage

  3. Clinical trials on human volunteers, first healthy then those with the illness
• Clinical trials

  Patients are randomly put into two groups - one gets the new drug, the other gets a placebo

  The trials are blind - neither the patient nor the doctor knows who is getting the drug or placebo

  The results are published only after peer review to prevent false claims
• Monoclonal antibodies

  Identical antibodies produced by cloning a single white blood cell

  They only target one specific antigen
• How monoclonal antibodies are produced

  Hybridoma cells are created by fusing a tumour cell with an antibody-producing cell

  The hybridoma cells can be grown to produce large quantities of the same monoclonal antibodies
• Uses of monoclonal antibodies

  • Used in pregnancy tests to detect the HCG hormone

  Used in research and labs to detect and measure specific substances
• Monoclonal antibodies do have some problems though, they can cause side effects
• Plant diseases

  Plants can be infected by microorganisms like fungi, bacteria and viruses

  Plants can also be damaged by pests
• Signs of plant diseases

  • Stunted growth

  Spots on leaves

  Patches of decay

  Abnormal growths

  Wilted or dropped leaves

  Discolouration