KEEPING HEALTHY - The bodies defence against infection

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

This page will answer many questions e.g.

How does our body defend itself when it becomes infected?

What are the physical and chemical methods of protection?

What is a pathogen?

What is our immune system?

What is a vaccine?   How does vaccination protect us?

How our bodies defend themselves against infectious diseases?

  • A simple example of how science works - cleanliness reduces the incidence of infection!

  • Be able to relate the contribution of Semmelweiss in controlling the rate of patient infection to solving modern problems with the spread of infection in hospitals.

    • Semmelweis worked in Vienna General Hospital in the 1840s and witnessed large numbers of women dying after childbirth from a puerperal fever disease.

    • He thought that the staff of the hospital were spreading the disease via unwashed hands.

    • After instructing doctors and nurses to wash their hands in an antiseptic solution, the mortality rate was considerably reduced.

    • Although Semmelweis didn't realise it at the time, the antiseptic solution was killing the infecting bacteria.

    • Apparently, when he left the Vienna hospital, the practice of washing hands in the antiseptic solution was relaxed, and the death rates rose again!

    • With the advent of new strain of bacteria today, there is now an even greater need for emphasis on hospital hygiene than ever before - so, if on a hospital visit, PLEASE WASH YOUR HANDS in the antiseptic gel provided.

  • Be able to explain how the treatment of disease has changed as a result of increased understanding of the action of antibiotics and immunity.

  • Be able to evaluate the consequences of mutations of bacteria and viruses in relation to epidemics and pandemics - data provided.

  • Be able to evaluate the advantages and disadvantages of being vaccinated against a particular disease - data provided.

    • In any evaluation questions data will be provided to work on.

  • Know that ...

    • a) Microorganisms that cause infectious disease are called pathogens.

    • b) Bacteria and viruses may reproduce rapidly inside the body and may produce poisons (toxins) that make us feel ill.

      • What is a bacteria? What is a pathogen? What is a virus?

      • Bacteria and certain protozoa are very small cells which can rapidly reproduce by cell division in your body making you feel ill by damaging your body's cells and producing toxins (poisons produced as a by-product of the bacteria's cell chemistry).

      • Viruses are NOT cells and much smaller than bacteria and damage the cells in which they reproduce.

        • Viruses replicate by invading a cell and using the cell's genetic machinery to reproduce themselves ie copies of the original virus.

        • The virus 'invaded' cell then bursts releasing lots of new viruses.

        • The cell damage makes you feel ill as your body (temporarily) fights back to make as many good cells as it can to replace those destroyed by the virus.

      • (Knowledge of the structure of bacteria and viruses is not required here.)

      • Fungi are also pathogens and includes microorganisms like yeasts and moulds (so don't eat mouldy food!).

    • c) The body has different physical and chemical ways of protecting itself against pathogens.

      • Physical protection from pathogens

        • Your skin and hairs and mucous in the respiratory tract can stop a lot of the pathogen cells from entering your body. The whole of the respiratory tract from the nasal passage, down the trachea and into the lungs is covered with mucous and lined cilia (fine hairs that can move freely at their ends). The mucous traps dust and bacteria before they can get down into the lungs and the cilia move the mucous along from the lungs up to the nasal passage -and then you can blow your nose!

        • Skin in good condition acts as a very effective barrier against pathogens. When a cut in the skin occurs, small sections of cells called platelets help the blood to clot quickly to seal the wound (seal = scab when dry) and prevent microorganisms entering the skin tissue or blood stream. The greater the concentration of platelets in the blood the faster the clotting process ('sealing') can occur.

      • Chemical protection by killing pathogens

        • In tears our eyes produce a chemical called lysozyme that kills bacterial microorganisms on the surface of the eye.

        • Your stomach contains quite concentrated hydrochloric acid which kills the majority of pathogenic bacteria - sadly not all of them at times!

    • d) White blood cells help to defend against pathogens by:

      • Ingesting pathogens

        • White cells can surround 'foreign' invasive microorganisms and break them up, effectively digesting them.

      • Producing antibodies, which destroy particular bacteria or viruses.

        • What is the function of white blood cells?

        • What is an antibody? antibodies? What is an antigen? What is an antitoxin?

        • All invading cells have unique molecules ('molecular structure') on their surface called antigens.

        • When white cells encounter a 'foreign' antigen on a pathogen they don't recognise, they produce proteins called antibodies which lock onto the antigens of the pathogen.

        • The antibodies produced are specific to that type of antigen, they will not lock onto any other type of antigen, hence they are specific to a particular pathogen.

        • One the 'blueprint' antibody is made, it is rapidly reproduced and they lock onto the invasive microbes and kill these bacteria.

          • A similar mechanism operates to fight viruses.

        • If a person becomes infected with the same microbic pathogen, the white blood cells will automatically, and quickly, produce the antibodies to kill the pathogen because of the first invasion of a particularly pathogen the person has become immune. This immunity helps prevent the immune person becoming ill again, or at least minimises the 2nd attack of the specific pathogen.

      • Producing antitoxins, which counteract the toxins released by the pathogens.

        • These antitoxins are very specific chemicals that remove the toxicity effect of the toxins produced by pathogen cell action.

      • White blood cells are one of the most important parts of the body's defences known as the immune system.

        • What is the immune system?

        • The immune system 'kicks in' if pathogens do get inside your body.

        • The white blood cells are present throughout your body in your blood system and therefore are always at hand to defend you from invading pathogens.

        • If your white blood cell count is low you are more susceptible to disease and infection.

        • For example, HIV/AIDS weakens white cell action and hence the body has a weaker responding immune system that allows pathogens to have a more devastating effect on the body - sometimes with fatal consequences from a disease that in a healthy body would not have proved fatal.

    • e) The immune system of the body produces specific antibodies to kill a particular pathogen.

      • This leads to immunity from that pathogen. In some cases, dead or inactivated pathogens stimulate antibody production.

      • If a large proportion of the population is immune to a pathogen, the spread of the pathogen is very much reduced.

    • f) Semmelweiss recognised the importance of hand-washing in the prevention of spreading some infectious diseases.

      • By insisting that doctors washed their hands before examining patients, he greatly reduced the number of deaths from infectious diseases in his hospital.

    • g) Some medicines, including painkillers, help to relieve the symptoms of infectious disease, but do not kill the pathogens.

    • h) Antibiotics, including penicillin, are medicines that help to cure bacterial disease by killing infectious bacteria inside the body.

      • What is an antibiotic?

      • Antibiotics cannot be used to kill viral pathogens, which live and reproduce inside cells.

        • Antibiotics do not destroy viruses, typified by the cold and flue viruses we all suffer from. Viruses make your own body cells reproduce the invasive virus and unfortunately anti-viral drugs may attack good cells too!

      • Antibiotics like penicillin kill or prevent the growth of harmful pathogens, they kill the bacteria but not your own body cells.

      • Different antibiotics attack different bacteria, so it is important that specific bacteria should be treated by specific antibiotics.

      • The use of antibiotics has greatly reduced deaths from infectious bacterial diseases.

      • However, overuse and inappropriate use of antibiotics has increased the rate of development of antibiotic resistant strains of bacteria.

      • You need to be aware that it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

      • Explaining the use of antibiotics to control infection:
      • Antibiotics are taken internally e.g. intravenous syringe injection, or orally taken tablet or liquid suspension.
        • Antibacterials to treat bacterial infections
        • Probably the most well known antibacterial is the antibiotic penicillin which is effective against many bacterial infections BUT NOT viruses like the common cold or flue.
        • An antibiotic can kill bacteria or prevent them growing and reproducing.
    • i) Many strains of bacteria, including MRSA, have developed resistance to antibiotics due to mutations, which cause stronger more resilient strains of bacteria to survive as a result of natural selection.

      • To prevent further resistance arising it is important to avoid over-use of antibiotics.

      • Knowledge of the development of resistance in bacteria is limited to the fact that pathogens mutate, producing resistant strains.

    • j) Mutations of pathogens produce new strains.

      • Antibiotics and vaccinations may no longer be effective against a new resistant strain of the pathogen.

      • The new strain will then spread rapidly because people are not immune to it and there is no effective treatment.

      • Can bacteria become resistant to antibiotics?

        • Unfortunately the answer is yes! Bacteria will sometimes quite naturally mutate into forms that are resistant to current antibiotics, so if your infected with a new strain of bacteria, your resistance is not as effective.

        • If an infection is treated with an antibiotic, any resistant bacteria will survive and this means resistant bacteria can survive and reproduce to infect other people, while the non-resistant strains will tend to be reduced.

        • This is an example of natural selection at the individual cell level and drug companies are constantly trying to develop new antibiotics to combat the new evolving strains of harmful bacteria - but new harmful 'superbugs' are becoming more common the more we use antibiotics and new epidemics can break out!

        • MRSA, methicillin-resistant staphylococcus aureus, can't be treated with many current antibiotics and causes serious wound infections that can be fatal to young babies or elderly people in particular.

        • Misuse by over-prescribing antibiotics is believed to be causing the rise of mutant resistant strains of bacteria, so doctors are being advised to avoid over-prescribing antibiotics to reduce the mutation rate and not treating mild infections with antibiotics.

        • It isn't just bacteria that can mutate, viruses can also evolve via new mutations. Viruses are notable for the rapidity with which they can mutate which makes it difficult to develop new vaccines. The reason being that changes in the virus (or bacteria) DNA leads to different gene expression in the form of different antigens, so different antibodies are needed. The flue virus is a never ending problem and in the past pandemics (epidemics across many countries at the same time) have killed millions of people, mercifully this rarely happens these days thanks to antibiotics.

      • HT only:  Understand that antibiotics kill individual pathogens of the non-resistant strain.

        • Individual resistant pathogens survive and reproduce, so the population of the resistant strain increases.

        • Now, antibiotics are not used to treat non-serious infections, such as mild throat infections, so that the rate of development of resistant strains is slowed down.

    • k) The development of antibiotic-resistant strains of bacteria necessitates the development of new antibiotics.

    • l) People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body (vaccination).

      • (c) doc b Know that vaccination is an important method of preventing infection.

      • What is vaccination? What is a vaccine?

      • Vaccination protects the individual from future infections and mass scale vaccination can greatly reduce the incidence of disease.

        • Protection is better than cure! If you become infected with a pathogen, it takes a few days for your white cell immune system to deal with the microorganism, and you can become quite ill in a few days.

      • Vaccination is the process of injecting the individual with small amounts of specific harmless dead/inactive microorganisms which carry the antigens that cause the immune system to produce the corresponding protective antibodies.

      • So, vaccines automatically stimulate the white blood cells to produce antibodies that destroy the invading 'foreign' pathogens.

      • This makes the person immune to future infections by the microorganism ie gives the individual immunity from further attacks.

      • The body can respond by rapidly making the correct antibody, in the same way as if the person had previously had the disease.

      • MMR vaccine is used to triple protect children against measles, mumps and rubella (German measles).

        • The vaccine contains weak inactive versions of three viruses that cause measles, mumps and rubella.

      • The effects of vaccination can 'wear off' over time, and booster injections maybe necessary to increase the levels of the protective antibodies.

      • There are arguments for and against vaccination (the 'pros and cons').

        • For: Vaccines have resulted in the large scale control of many infectious diseases that were once common and often fatal eg measles, mumps, polio, rubella, smallpox, tetanus, whooping cough etc. Epidemics are less likely with mass vaccination - spread of the disease is less likely as there are fewer infected people carry an active form of the disease. Without mass vaccination an outbreak of epidemic proportions is much more likely - more people potentially to carry the disease.

        • Against: Some vaccines do not always give you immunity but development work goes on all the time to make more effective vaccines - especially as different strains of viruses and bacteria are constantly evolving. There may also be side-effects in which the 'patient' has a bad reaction to a particular vaccine eg swelling, fever, seizure (serious!), but such reactions and complications are rare and the mass good effect is balanced against the very rare negative effect.

          • There is a very small risk involved with most medical treatments and although side-effects are not uncommon, without vaccination some of these diseases are fatal or have very serious non-fatal outcomes - people can die of from measles, rubella has serious consequences for pregnant women, there can be serious complications for infected people who have not been vaccinated.

          • Following a seaside accident, as an eleven year old, I collapsed unconscious after a tetanus injection at a local hospital. I was ok within half an hour BUT my parents got a bit of a shock!

      • Details of vaccination schedules and side effects associated with specific vaccines are not required.

    • m) Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.

      • For this:

        • Petri dishes and culture media must be sterilised before use to kill unwanted microorganisms.

        • Inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame.

        • The lid of the Petri dish should be secured with adhesive tape to prevent microorganisms from the air contaminating the culture.

    • n) In school and college laboratories, cultures should be incubated at a maximum temperature of 25 °C, which greatly reduces the likelihood of growth of pathogens that might be harmful to humans.

    • o) In industrial conditions higher temperatures can produce more rapid growth of unwanted, potentially harmful microorganisms.

    • Any practical work and investigations you did should also be revised - good context material for exam questions! See below!

  • Hopefully your school practical work will include the following (which should also be revised, helps in understanding 'how science works' and context examination questions):

    • Investigate the effectiveness of various antibiotic discs in killing bacteria.

    • Growing microorganisms in Petri dishes to demonstrate sterile technique and growing pure cultures.

      • Microorganisms are cultured in a culture medium which usually consists of agar jelly containing carbohydrates, minerals, proteins and vitamins that supply all the nutrients needed for cell growth.

    • Using pre-inoculated agar in Petri dishes to evaluate the effect of disinfectants and antibiotics.

      • The hot liquid agar jelly is poured into shallow Petri dishes to cool and set - just like a jelly!

      • The Petri dishes should be covered with an air-tight lid to stop microorganisms from the surrounding air contaminating the experiments.

      • Wire loops, sterilised in a hot flame, are used to transfer microorganisms onto the agar jelly, where they multiply producing many colonies quite rapidly - lots to feed on!

        • If the wire loop is not sterilised, other bacteria may contaminate the experiment and these other microorganisms will confuse the results.

      • Little bits of porous paper (filter paper?) soaked in different antibiotics are placed on the bacterial colonies on the jelly.

      • You can then see which antibiotics kill what bacteria, but the antibiotic-resistant bacteria will continue to grow.

      • In the pharmaceutical and medical industries where extremely dangerous pathogens are being investigated, extremely strict health and safety regulation is essential for both the safety of workers and members of the general public.


  • Seeing computer simulations to model

    • the effect of ...

      • (i) the growth of bacterial colonies in varying conditions,

      • (ii) the action of the immune system and the effect of antibiotics and vaccines.


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