B3 - Infection + Response

• Pathogens


  Microorganisms that enter the body and cause disease
• Pathogens


  • They cause communicable (infectious) diseases
  • They can infect both plants and animals
• Bacteria


  Very small living cells (about 1/100th the size of your body cells), which can reproduce rapidly inside your body
• Bacteria


  They can make you feel ill by producing toxins (poisons) that damage your cells and tissues
• Viruses


  • They're tiny, about 1/100th the size of a bacterium
  • They live inside your cells and replicate themselves using the cells' machinery to produce many copies of themselves
  • The cell will usually then burst, releasing all the new viruses
• Viruses


  This cell damage is what makes you feel ill
• Protists


  • They're all eukaryotes and most of them are single-celled
  • Some protists are parasites that live on or inside other organisms and can cause them damage
  • They are often transferred to the organism by a vector, which doesn't get the disease itself
• Fungi


  • Some are single-celled, others have a body made up of hyphae (thread-like structures)
  • The hyphae can grow and penetrate human skin and the surface of plants, causing diseases
  • The hyphae can produce spores, which can be spread to other plants and animals
• How pathogens can be spread

  1. Water

  2. Air

  3. Direct contact
• Viral diseases


  • Measles
  • HIV
  • Tobacco mosaic virus (TMV)
• Fungal disease


  • Rose black spot
• Disease caused by a protist

  • Malaria
• Bacterial diseases


  • Salmonella
  • Gonorrhoea
• Ways to reduce or prevent the spread of disease

  1. Being hygienic

  2. Destroying vectors

  3. Isolating infected individuals

  4. Vaccination
• Human body's defence system

  • Skin acts as a barrier to pathogens
  • Hairs and mucus in nose trap particles
  • Trachea and bronchi secrete mucus and have cilia to trap pathogens
  • Stomach produces hydrochloric acid to kill pathogens
• How the immune system attacks pathogens

  1. Consuming them (phagocytosis)

  2. Producing antibodies

  3. Producing antitoxins
• Antibodies are produced rapidly and carried around the body to find all similar bacteria or viruses
• If the person is infected with the same pathogen again the white blood cells will rapidly produce the antibodies to kill it - the person is naturally immune to that pathogen and won't get ill
• Producing Antibodies


  1. Every invading pathogen has unique molecules (called antigens) on its surface

  2. When some types of white blood cell come across a foreign antigen, they will start to produce proteins called antibodies to lock onto the invading cells so that they can be found and destroyed by other white blood cells

  3. The antibodies produced are specific to that type of antigen- they won't lock on to any others

  4. Antibodies are then produced rapidly and carried around the body to find all similar bacteria or viruses

  5. If the person is infected with the same pathogen again the white blood cells will rapidly produce the antibodies to kill it the person is naturally immune to that pathogen and won't get ill
• New pathogens


  Attacked by new antibodies
• The white blood cells that produce antibodies are also known as B-lymphocytes
• Producing Antitoxins


  These counteract toxins produced by the invading bacteria
• Vaccinations Can Protect from Future Infections

  1. When you're infected with a new pathogen, it takes your white blood cells a few days to learn how to deal with it

  2. Vaccinations involve injecting small amounts of dead or inactive pathogens. These carry antigens, which cause your body to produce antibodies to attack them even though the pathogen is harmless (since it's dead or inactive)

  3. But if live pathogens of the same type appear after that, the white blood cells can rapidly mass-produce antibodies to kill off the pathogen
• Some Drugs Relieve Symptoms - Others Cure the Problem

  1. Painkillers (e.g. aspirin) are drugs that relieve pain, but they don't actually tackle the cause of the disease or kill pathogens, they just help to reduce the symptoms

  2. Antibiotics (e.g. penicillin) actually kill (or prevent the growth of) the bacteria causing the problem without killing your own body cells

  3. Antibiotics don't destroy viruses (e.g. flu or cold viruses)
• The use of antibiotics has greatly reduced the number of deaths from communicable diseases caused by bacteria
• Bacteria Can Become Resistant to Antibiotics

  1. Bacteria can mutate (change). This can cause them to be resistant to (not killed by) an antibiotic

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

  3. This means that when you treat the infection, only the non-resistant strains of bacteria will be killed

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

  5. To slow down the rate of development of resistant strains, it's important for doctors to avoid over-prescribing antibiotics

  6. It's also important that you finish the whole course of antibiotics and don't just stop once you feel better
• E.g. MRSA (meticillin-resistant Staphylococcus aureus) causes serious wound infections and is resistant to the powerful antibiotic meticillin
• Many Drugs Originally Came From Plants

  1. Plants produce a variety of chemicals to defend themselves against pests and pathogens

  2. Some of these chemicals can be used as drugs to treat human diseases or relieve symptoms

  3. Aspirin is used as a painkiller and to lower fever. It was developed from a chemical found in willow

  4. Digitalis is used to treat heart conditions. It was developed from a chemical found in foxgloves

  5. Alexander Fleming was clearing out some Petri dishes containing bacteria. He noticed that one of the dishes of bacteria also had mould on it and the area around the mould was free of the bacteria. He found that the mould (called Penicillium notatum) on the Petri dish was producing a substance that killed the bacteria - this substance was penicillin

  6. These days, drugs are made on a large scale in the pharmaceutical industry - they're synthesised by chemists in labs. However, the process still might start with a chemical extracted from a plant
• There are Three Main Stages in Drug Testing

  1. In preclinical testing, drugs are tested on human cells and tissues in the lab

  2. The next step in preclinical testing is to test the drug on live animals

  3. If the drug passes the tests on animals then it's tested on human volunteers in a clinical trial
• The law in Britain states that any new drug must be tested on two different live mammals
• Clinical Trials


  1. First, the drug is tested on healthy volunteers. This is to make sure that it doesn't have any harmful side effects when the body is working normally

  2. If the results of the tests on healthy volunteers are good, the drugs can be tested on people suffering from the illness

  3. To test how well the drug works, patients are randomly put into two groups. One is given the new drug, the other is given a placebo

  4. Clinical trials are blind - the patient in the study doesn't know whether they're getting the drug or the placebo. In fact, they're often double-blind - neither the patient nor the doctor knows until all the results have been gathered
• The results of drug testing and drug trials aren't published until they've been through peer review
• Peer review


  When other scientists check that the work is valid and has been carried out rigorously