Pathogens and the immune system
By the end of this chapter you will be able to
- Name the four types of pathogen and give an example of each
- Explain how the skin, nose and stomach protect against pathogens
- Describe how phagocytes and lymphocytes respond to a pathogen
- Explain the immune response to a new pathogen, including memory cells
- Explain how diseases spread, and describe the effects of HIV, measles and malaria
You should already know
- Cells and their parts (Chapter 2)
- White blood cells and antibodies (Chapter 6)
- Enzymes and the stomach (Chapter 5)
Where this leads
- Vaccination, antibiotics and drugs (Chapter 14)
- Cell division in the immune response (Chapter 4)
- Natural selection of resistant bacteria (Chapter 16)
GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the four types of pathogen and how they cause disease, Describe the body’s first barriers and the white blood cells, Explain antigens, antibodies and antitoxins, and the immune response to a new pathogen, Explain memory cells and why a second infection is dealt with faster, Describe how diseases spread, with HIV, measles and malaria as examples.
Exercise P Do you remember?
Method. Recall the cell that carries oxygen.
- Recall. Red blood cells carry oxygen using haemoglobin.carry oxygen
- Write the answer. red blood cells
Check. White blood cells defend the body. ✓
Method. Recall the defence cells.
- Recall. White blood cells defend the body against pathogens.defend the body
- Write the answer. white blood cells
Check. Platelets help blood clot. ✓
Method. Multiply 3 by 9.
- Multiply. 3 x 9 = 27.3 x 9 = 27
- Write the answer. 27
Check. 27 divided by 9 is 3. ✓
Method. Recall the organ.
- Recall. Bile is made in the liver.made in the liver
- Write the answer. liver
Check. Bile is stored in the gall bladder. ✓
Method. Recall the white blood cell.
- Recall. A white blood cell that engulfs pathogens is a phagocyte.engulfs pathogens
- Write the answer. phagocyte
Check. Phagocytes are one type of white blood cell. ✓
Pathogens and how they cause disease
A pathogen is a microorganism that causes disease. The four main types are bacteria, viruses, protists and fungi. Bacteria are single-celled and often make toxins that damage the body. Viruses are not cells: they reproduce only inside the cells of their host, damaging them. Protists are single-celled organisms such as Plasmodium, which causes malaria. Fungi include the organisms that cause athlete’s foot.
| Type | Example | How it spreads |
|---|---|---|
| bacteria | salmonella (food poisoning) | contaminated food and water |
| virus | measles | droplets in the air from coughs and sneezes |
| protist | malaria (Plasmodium) | a mosquito, which acts as a vector |
| fungus | athlete’s foot | direct contact, for example in a shared shower |
Key ideas
- A pathogen is a microorganism that causes disease.
- Bacteria are single-celled; many release toxins that damage the body.
- Viruses are not cells; they reproduce inside host cells and damage them.
- Protists and fungi are other pathogen types; malaria is caused by a protist.
Method. Recall the non-cellular pathogen.
- Recall. Viruses are not cells, and reproduce only inside the cells of their host.inside the cells
- Name it. So the pathogen is a virus.virus
- Write the answer. virus
Check. Measles is caused by a virus. ✓
Method. Recall the protist.
- Recall. Malaria is caused by a protist, Plasmodium.a protist
- Name it. So the pathogen is Plasmodium, a protist.Plasmodium
- Write the answer. Plasmodium (a protist)
Check. Plasmodium is spread by a mosquito. ✓
Method. Recall the toxin maker.
- Recall. Many bacteria release toxins that damage the body.release toxins
- Name it. So the type is bacteria.bacteria
- Write the answer. bacteria
Check. Salmonella is a bacterium that causes food poisoning. ✓
Watch out
Watch out: antibiotics kill bacteria, but not viruses. Do not assume that every infection can be treated in the same way (Chapter 14 covers this).
Exam tip
Exam tip. Name the pathogen type and give one example, for example “virus: measles”.
Try it (with your teacher). With your teacher, use a labelled picture or a model to compare a bacterium and a virus. Write one difference in how each reproduces. Do not grow or handle any microorganisms.
Exercise 13A Pathogens and how they cause disease
Method. Recall the four types.
- Recall. The four main types are bacteria, viruses, protists and fungi.four main types
- Write the answer. bacteria, viruses, protists and fungi
Check. These are all microorganisms. ✓
Method. Recall which pathogen is not a cell.
- Recall. Viruses are not cells.not cells
- Write the answer. virus
Check. Bacteria are single-celled. ✓
Method. Recall the cause of malaria.
- Recall. Malaria is caused by a protist.caused by a protist
- Write the answer. protist
Check. Athlete’s foot is caused by a fungus. ✓
Method. Recall the fungus.
- Recall. Athlete’s foot is caused by a fungus.caused by a fungus
- Write the answer. fungus
Check. Fungi are not bacteria. ✓
Method. Recall the toxin-making pathogen.
- Recall. Many bacteria release toxins that damage the body.release toxins
- Write the answer. bacteria
Check. Toxins are poisons made by the pathogen. ✓
Method. Recall the reproduction of viruses.
- Recall. Viruses reproduce only inside the cells of their host.inside the cells of their host
- Write the answer. virus
Check. Viruses damage the host cells. ✓
Method. Recall the cause of measles.
- Recall. Measles is caused by a virus.caused by a virus
- Write the answer. virus
Check. Measles spreads in droplets. ✓
Method. Recall what a virus is.
- Recall. A virus is not a cell.not a cell
- Write the answer. So no.no
- Write the answer. no
Check. Viruses need a host cell to reproduce. ✓
Method. Recall the single-celled group.
- Recall. Plasmodium is a single-celled organism in the protist group.protist group
- Name it. So the type is protist.protist
- Write the answer. protist
Check. Protists are not bacteria. ✓
Method. Think about the target of antibiotics.
- Think. Antibiotics act on bacteria, not on viruses.act on bacteria
- Conclude. So they do not treat a cold caused by a virus.do not treat
- Write the answer. Accept any sentence that says antibiotics kill bacteria, but not viruses.
Check. Viruses reproduce inside cells, out of reach of antibiotics. ✓
Method. Think about where the virus is.
- Location. A virus reproduces inside the body’s own cells.inside the body’s own cells
- Treatment. A treatment must act on the virus without harming those cells, which is hard.without harming
- Conclude. Antibiotics do not act on viruses, so the choice of treatment is limited.limited
- Write the answer. Accept any answer that says a virus lives inside the host cells, so drugs must act without harming them, and antibiotics do not target it.
Check. The virus is hidden inside the cells. ✓
Barriers and white blood cells
The body’s first defences are barriers. The skin is a physical barrier that pathogens cannot easily cross. The nose has hairs and mucus that trap dust and microorganisms. The stomach produces acid that kills most pathogens that are swallowed.
If pathogens get past the barriers, white blood cells respond. Phagocytes engulf pathogens and digest them, a process called phagocytosis. Lymphocytes produce antibodies, and they are specific to one antigen.
| Barrier | How it protects |
|---|---|
| skin | a physical barrier that pathogens cannot easily cross |
| nose hairs and mucus | trap dust and microorganisms before they reach the lungs |
| stomach acid | kills most pathogens that are swallowed |
Key ideas
- The skin, nose and stomach are the first barriers.
- Mucus and nose hairs trap dust and microorganisms.
- Phagocytes engulf and digest pathogens (phagocytosis).
- Lymphocytes produce antibodies specific to a pathogen.
Method. Recall the process.
- Recall. A phagocyte surrounds the pathogen and digests it.surrounds the pathogen
- Name it. This process is phagocytosis.phagocytosis
- Write the answer. phagocytosis
Check. Phagocytes are a type of white blood cell. ✓
Method. Recall the stomach.
- Recall. The stomach produces acid that kills most pathogens.produces acid
- Name it. So the barrier is stomach acid.stomach acid
- Write the answer. stomach acid
Check. The skin is another barrier. ✓
Method. Recall the nose defences.
- Recall. Hairs and mucus in the nose trap dust and microorganisms.hairs and mucus
- Name one. So nose hairs (or the mucus) are the defence.nose hairs
- Write the answer. nose hairs (or mucus)
Check. Mucus is also produced in the airways. ✓
Watch out
Watch out: phagocytes engulf and digest pathogens. Lymphocytes do not engulf them; they make antibodies. Do not mix the two jobs.
Exam tip
Exam tip. Name the cell type and then its job: “phagocyte: engulfs and digests pathogens”.
Try it (with your teacher). With your teacher, look at a labelled diagram of the skin and nose. List the parts that act as barriers, and say how each one stops a pathogen.
Exercise 13B Barriers and white blood cells
Method. Recall the outer barrier.
- Recall. The skin is a physical barrier over the body.physical barrier
- Write the answer. skin
Check. Pathogens cannot easily cross intact skin. ✓
Method. Recall the trapping substance.
- Recall. Mucus in the nose traps dust and microorganisms.traps dust
- Write the answer. mucus
Check. The nose hairs also trap particles. ✓
Method. Recall the stomach acid.
- Recall. The stomach produces hydrochloric acid, which kills most swallowed pathogens.hydrochloric acid
- Write the answer. hydrochloric acid
Check. The stomach acid also helps digestion. ✓
Method. Recall the cell.
- Recall. A phagocyte engulfs and digests pathogens.engulfs and digests
- Write the answer. phagocyte
Check. Lymphocytes make antibodies. ✓
Method. Recall the process.
- Recall. Phagocytosis is the engulfing and digesting of a pathogen.engulfing and digesting
- Write the answer. phagocytosis
Check. Phagocytes carry out phagocytosis. ✓
Method. Recall the antibody producers.
- Recall. Lymphocytes produce antibodies.produce antibodies
- Write the answer. lymphocytes
Check. Phagocytes do not make antibodies. ✓
Method. Think about a cut.
- Think. A cut breaks the skin, which lets pathogens in.breaks the skin
- Conclude. So the barrier is broken, and the skin must heal.must heal
- Write the answer. Accept skin.
Check. Clotting and healing close the gap. ✓
Method. Think about what is swallowed.
- Think. Pathogens in food and drink are swallowed into the stomach.swallowed
- Conclude. The acid kills most of them before they can get into the body.before they can get in
- Write the answer. Accept any sentence that says the acid kills most pathogens that are swallowed, so they cannot infect the body.
Check. The acid is a chemical barrier. ✓
Method. Compare the jobs.
- Phagocyte. A phagocyte engulfs and digests the pathogen.engulfs and digests
- Lymphocyte. A lymphocyte produces antibodies that mark the pathogen.produces antibodies
- Write the answer. Accept any sentence that says the lymphocyte makes antibodies instead of engulfing the pathogen.
Check. Two different defences work together. ✓
Method. Describe the two lines of defence.
- Barriers. Barriers stop most pathogens from ever entering the body.stop most pathogens
- Cells. White blood cells deal with the pathogens that do get past the barriers.deal with
- Conclude. Together they give two lines of defence.two lines
- Write the answer. Accept any answer that says barriers stop most pathogens entering, and white blood cells deal with any that get past them.
Check. Defence in depth is a useful idea. ✓
Antigens, antibodies and the immune response
An antigen is a molecule on the surface of a pathogen that the body recognises as foreign. When lymphocytes meet a pathogen with an antigen that matches their receptors, they become active and clone themselves. Each clone produces antibodies, which are specific to that antigen, and the antibodies bind to the pathogen to mark it for destruction.
Some pathogens produce toxins. Lymphocytes can make antitoxins, which neutralise the toxins.
Key ideas
- An antigen is a molecule on a pathogen that is recognised as foreign.
- Each antibody fits one specific antigen.
- Lymphocytes clone themselves when they meet the matching antigen.
- Antitoxins neutralise the toxins made by some pathogens.
Method. Recall the recognised molecule.
- Recall. The molecule on a pathogen that the body recognises is an antigen.recognises
- Name it. So the word is antigen.antigen
- Write the answer. antigen
Check. Each pathogen has its own antigens. ✓
Method. Recall the binding molecules.
- Recall. Lymphocytes make antibodies, which bind to specific antigens.bind to specific antigens
- Name it. So the molecules are antibodies.antibodies
- Write the answer. antibodies
Check. Antibodies are specific to one antigen. ✓
Method. Recall the toxin neutraliser.
- Recall. Antitoxins neutralise the toxins made by some pathogens.neutralise the toxins
- Name it. So the substances are antitoxins.antitoxins
- Write the answer. antitoxins
Check. Antitoxins act against toxins, not pathogens directly. ✓
Watch out
Watch out: an antibody is specific to one antigen. It does not fit every pathogen. The lymphocyte must match the antigen first.
Exam tip
Exam tip. When you explain the immune response, say that the lymphocyte is specific to the antigen, and that it clones itself to make many antibodies.
Try it (with your teacher). With your teacher, use two different shaped objects and matching holes in a card. Show how a shape fits only its matching hole, and link it to an antibody and an antigen.
Exercise 13C Antigens, antibodies and the immune response
Method. Recall the definition.
- Recall. An antigen is a molecule on a pathogen that the body recognises as foreign.recognised as foreign
- Write the answer. a molecule on a pathogen recognised as foreign
Check. Antigens trigger an immune response. ✓
Method. Recall the cells.
- Recall. Lymphocytes make antibodies.make antibodies
- Write the answer. lymphocytes
Check. Phagocytes engulf pathogens. ✓
Method. Recall the specificity.
- Recall. Each antibody fits one specific antigen.fits one specific antigen
- Write the answer. So yes.yes
- Write the answer. yes
Check. Specificity is why the lock and key idea applies. ✓
Method. Recall the job.
- Recall. Antitoxins neutralise the toxins that some pathogens produce.neutralise the toxins
- Write the answer. neutralise toxins
Check. They do not kill the pathogen itself. ✓
Method. Recall the response of lymphocytes.
- Recall. Lymphocytes that match an antigen divide, making clones of themselves.making clones
- Write the answer. clone
Check. The clones produce antibodies. ✓
Method. Recall the first response.
- Recall. The first response to a new pathogen is slow, because few lymphocytes match the antigen.is slow
- Write the answer. slowly
Check. The second response is much faster. ✓
Method. Compare the two curves.
- Compare. The second exposure rises to 8 by day 2, while the first is only 1.8 and 1
- Name it. So the second exposure rises faster.second exposure
- Write the answer. the second exposure
Check. Memory cells cause the faster rise. ✓
Method. Read the blue curve.
- Read. The first exposure is at 8 at day 8.8
- Write the answer. 8
Check. The second exposure is 30 at the same time. ✓
Method. Read the red curve.
- Read. The second exposure is at 20 at day 4.20
- Write the answer. 20
Check. The first exposure is only 3 at day 4. ✓
Method. Think about the shape.
- Think. An antibody has a shape that fits one specific antigen, like a lock and key.lock and key
- Conclude. So it binds to that antigen only.that antigen only
- Write the answer. Accept any sentence that says its shape fits one specific antigen.
Check. The shape is specific. ✓
Method. Link the number of matching lymphocytes to the time.
- Few. At first only a few lymphocytes match the new antigen.only a few
- Clone. These must divide to make enough clones to produce many antibodies.divide to make
- Write the answer. Accept any answer that says few lymphocytes match the antigen at first, so they must clone before enough antibodies are made.
Check. Time is needed for cloning. ✓
Memory cells and the second infection
After the first infection, some of the lymphocytes that matched the antigen become memory cells. They stay in the blood for a long time. If the same pathogen enters the body again, the memory cells recognise the antigen quickly.
The second response is faster, starts sooner, and produces more antibodies, so the pathogen is destroyed before it causes illness. This is immunity.
Key ideas
- Memory cells remain after an infection, to recognise the antigen again.
- A second response is faster and produces more antibodies.
- Immunity is the body’s ability to destroy a pathogen before it causes illness.
- Memory cells are the reason many diseases are caught only once.
Method. Recall the cells that stay.
- Recall. Some lymphocytes that matched the antigen remain in the blood as memory cells.remain as memory cells
- Name them. So the cells are memory cells.memory cells
- Write the answer. memory cells
Check. They recognise the antigen again. ✓
Method. Think about the memory cells.
- Think. Memory cells already recognise the antigen.already recognise
- Conclude. So they quickly clone and produce antibodies, which is faster than the first time.quickly clone
- Write the answer. Memory cells already recognise the antigen, so they clone quickly and produce antibodies faster than the first time.
Check. Memory cells reduce the time to respond. ✓
Method. Recall the state.
- Recall. The body is protected against the pathogen, which is immunity.protected
- Name it. So the state is immunity.immunity
- Write the answer. immunity
Check. Immunity means the pathogen is destroyed before illness. ✓
Watch out
Watch out: memory cells do not stop the first infection. They make the second response faster; the first response is always slower.
Exam tip
Exam tip. When you compare two responses, say which is faster and which produces more antibodies, and give the reason: memory cells.
Try it (with your teacher). With your teacher, plot the two curves of the antibody graph on squared paper. Mark the points where the second response is at least five times the first.
Exercise 13D Memory cells and the second infection
Method. Recall the cells.
- Recall. Memory cells remain in the blood after an infection.remain in the blood
- Write the answer. memory cells
Check. Memory cells are a type of lymphocyte. ✓
Method. Compare the responses.
- Compare. Memory cells make the second response start sooner.start sooner
- Decide. So it is faster.faster
- Write the answer. faster
Check. The second response also produces more antibodies. ✓
Method. Check the amounts.
- Check. The second exposure reaches 30 units; the first reaches 8.30 and 8
- Write the answer. So yes, the second response produces more antibodies.yes
- Write the answer. yes
Check. The amounts show the difference. ✓
Method. Recall the term.
- Recall. The body’s ability to destroy a pathogen before illness is immunity.ability to destroy
- Write the answer. immunity
Check. Memory cells help give immunity. ✓
Method. Read the blue curve.
- Read. The first exposure is at 6 at day 6.6
- Write the answer. 6
Check. It is lower than the second exposure. ✓
Method. Read the red curve.
- Read. The second exposure is at 30 at day 6.30
- Write the answer. 30
Check. It is five times the first at this point. ✓
Method. Divide the two levels.
- Read. The second is 30 and the first is 6.30 and 6
- Divide. 30 / 6 = 5.30 / 6 = 5
- Write the answer. 5
Check. 6 x 5 = 30. ✓
Method. Find the first point at 20 or more.
- Look. At day 2 the level is 8; at day 4 it is 20.day 4
- Name it. So day 4 is the first day at 20 or more.4
- Write the answer. 4
Check. Day 4 is 20 and day 2 is 8. ✓
Method. Link the memory cells to the protection.
- Memory. Memory cells from the first infection stay in the blood.stay in the blood
- Respond. If the same pathogen returns, they recognise the antigen and respond quickly.respond quickly
- Write the answer. Accept any sentence that says memory cells from the first infection recognise the antigen and respond quickly.
Check. The second response destroys the pathogen early. ✓
Method. Think about what a vaccine contains.
- Think. A vaccine contains an antigen, but not the full disease.not the full disease
- Conclude. So it makes memory cells, without the illness.without the illness
- Write the answer. Accept any sentence that says it triggers lymphocytes to make memory cells without causing the illness.
Check. Vaccination is covered in Chapter 14. ✓
Method. Link memory cells to the peak.
- Memory. Memory cells already match the antigen, so antibodies are made more quickly.more quickly
- Peak. More cells are ready, so the peak antibody level is higher.peak is higher
- Conclude. The pathogen is destroyed before it can cause illness.before it can cause illness
- Write the answer. Accept any answer that says memory cells produce more antibodies faster, so the peak is higher and the pathogen is destroyed more quickly.
Check. The two effects come from the memory cells. ✓
How diseases spread: HIV, measles and malaria
Diseases spread by direct contact (touching an infected person), by water and food, by air (droplets from coughs and sneezes), and by a vector, an organism that carries a pathogen from one host to another. Malaria is spread by a mosquito, which is a vector.
HIV is a virus that attacks lymphocytes, weakening the immune system. Over time, the person can develop AIDS, in which many infections become hard to fight. HIV is spread by body fluids, including through unprotected sex and shared needles.
Measles is a viral disease spread by droplets in the air, causing fever and a rash. Malaria is caused by the protist Plasmodium, which is spread by mosquitoes. It damages red blood cells and causes fever.
| Way of spreading | Example |
|---|---|
| direct contact | touching an infected person or a shared towel |
| water and food | contaminated drinking water or food |
| air (droplets) | coughs and sneezes that carry a virus |
| vector | a mosquito that carries the protist that causes malaria |
Key ideas
- Diseases spread by direct contact, water and food, air, or a vector.
- HIV attacks lymphocytes and weakens the immune system.
- Measles is a viral disease spread by droplets in the air.
- Malaria is caused by a protist, and it is spread by a mosquito.
Method. Recall the carrier.
- Recall. An organism that carries a pathogen from one host to another is a vector.carries a pathogen
- Name it. So the word is vector.vector
- Write the answer. vector
Check. A mosquito is a vector for malaria. ✓
Method. Recall the cells attacked by HIV.
- Recall. HIV attacks lymphocytes, which are the cells that fight infection.attacks lymphocytes
- Name them. So the cells are lymphocytes.lymphocytes
- Write the answer. lymphocytes
Check. Weakening these cells makes AIDS possible. ✓
Method. Recall the spread of measles.
- Recall. Measles spreads by droplets in the air from coughs and sneezes.droplets in the air
- Name it. So it spreads through the air.through the air
- Write the answer. through the air (droplets)
Check. Measles is a virus. ✓
Watch out
Watch out: HIV is not spread by casual contact such as shaking hands or sharing a classroom. Malaria is not spread directly from person to person; it needs a mosquito.
Exam tip
Exam tip. When you describe the effect of a disease, name the cell or system it affects, and say what happens to the body as a result.
Try it (with your teacher). With your teacher, sort a list of diseases into the ways they spread: direct contact, water and food, air, or a vector. Use only the list in the chapter, and check your answers with your teacher.
Exercise 13E How diseases spread: HIV, measles and malaria
Method. Recall the spread of malaria.
- Recall. Malaria is spread by mosquitoes, which act as vectors.spread by mosquitoes
- Write the answer. by a mosquito (a vector)
Check. The protist is carried by the mosquito. ✓
Method. Recall the virus.
- Recall. HIV is the virus that attacks lymphocytes.attacks lymphocytes
- Write the answer. HIV
Check. AIDS is the late stage of HIV infection. ✓
Method. Recall the condition.
- Recall. AIDS is the condition in which the immune system is badly weakened by HIV.badly weakened
- Write the answer. AIDS
Check. HIV is the virus; AIDS is the condition. ✓
Method. Recall the symptom.
- Recall. Measles causes a fever and a rash.a rash
- Write the answer. rash
Check. Measles is spread by droplets. ✓
Method. Recall the protist.
- Recall. Malaria is caused by the protist Plasmodium.Plasmodium
- Write the answer. Plasmodium
Check. The mosquito carries it. ✓
Method. Recall the cells damaged.
- Recall. Plasmodium damages red blood cells.damages red blood cells
- Write the answer. red blood cells
Check. Red blood cells carry oxygen. ✓
Method. Compare the spread of HIV and measles.
- Measles. Measles is spread by droplets in the air.droplets in the air
- HIV. HIV is spread through body fluids, not through the air.body fluids
- Write the answer. So no.no
- Write the answer. no
Check. Different diseases have different routes. ✓
Method. Think about the water supply.
- Think. Water can be contaminated with a pathogen.contaminated
- Conclude. Drinking it spreads the pathogen to many people.many people
- Write the answer. Accept any sentence that says contaminated drinking water can carry a pathogen to many people.
Check. Clean water supplies prevent this. ✓
Method. Think about the vector.
- Think. Mosquitoes carry the protist that causes malaria.carry the protist
- Conclude. A net stops the mosquito reaching the person, so the protist cannot be passed on.cannot be passed on
- Write the answer. Accept any sentence that says it stops mosquitoes reaching the person, so the vector cannot pass on the protist.
Check. Breaking the vector link reduces the spread. ✓
Method. Compare the two effects.
- HIV. HIV weakens the lymphocytes, so the body cannot fight other pathogens.cannot fight other pathogens
- Measles. Measles causes its own illness, with a fever and rash.its own illness
- Conclude. The HIV damage is a weakened defence, not a single illness.weakened defence
- Write the answer. Accept any answer that says HIV weakens the immune system so other pathogens cause infections, while measles causes its own illness.
Check. Compare the mechanism, not just the symptoms. ✓
Chapter summary
| Skill | Method |
|---|---|
| Pathogens | Bacteria, viruses, protists and fungi; each causes disease in a different way. |
| Barriers | Skin, mucus and nose hairs, and stomach acid. |
| White blood cells | Phagocytes engulf pathogens (phagocytosis); lymphocytes make antibodies and antitoxins. |
| Antigens and antibodies | Each antibody fits one specific antigen; lymphocytes clone when they meet a match. |
| Memory cells | Remain after an infection, so the second response is faster and bigger. |
| Spread | Direct contact, water and food, air (droplets), or a vector such as a mosquito. |
| HIV, measles, malaria | HIV attacks lymphocytes; measles is a viral disease spread by droplets; malaria is a protist spread by mosquitoes. |
Can you…?
- I can name the four types of pathogen and give an example of each
- I can explain how the skin, nose and stomach protect against pathogens
- I can describe how phagocytes and lymphocytes respond to a pathogen
- I can explain the immune response to a new pathogen and the role of memory cells
- I can explain how diseases spread, with HIV, measles and malaria as examples
Key terms
- pathogen
- a microorganism that causes disease
- toxin
- a poison made by a pathogen that damages the body
- vector
- an organism that carries a pathogen from one host to another
- phagocytosis
- the engulfing and digesting of a pathogen by a phagocyte
- antigen
- a molecule on a pathogen that the body recognises as foreign
- antibody
- a protein made by lymphocytes that binds to a specific antigen
- antitoxin
- a substance made by lymphocytes that neutralises a toxin
- memory cell
- a lymphocyte that remains after an infection and recognises the antigen again
- immunity
- the body’s ability to destroy a pathogen before it causes illness
- AIDS
- the condition in which the immune system is badly weakened by HIV
Exercise 13Z Check-up
Method. Recall the four types.
- Recall. The four types are bacteria, viruses, protists and fungi.four types
- Write the answer. bacteria, viruses, protists and fungi
Check. All are microorganisms. ✓
Method. Recall the process.
- Recall. Phagocytosis is the engulfing and digesting of a pathogen.engulfing and digesting
- Write the answer. phagocytosis
Check. Phagocytes carry this out. ✓
Method. Recall the molecule.
- Recall. An antigen is a molecule on a pathogen that is recognised as foreign.recognised as foreign
- Write the answer. antigen
Check. Antibodies bind to antigens. ✓
Method. Recall the cells.
- Recall. Memory cells remain after an infection.remain after an infection
- Write the answer. memory cells
Check. They make the second response faster. ✓
Method. Recall the disease.
- Recall. Malaria is caused by a protist and spread by mosquitoes.spread by mosquitoes
- Write the answer. malaria
Check. The mosquito is a vector. ✓
Method. Recall the virus.
- Recall. HIV attacks lymphocytes.attacks lymphocytes
- Write the answer. HIV
Check. AIDS is the condition it can cause. ✓
Method. Recall the barrier.
- Recall. The stomach produces acid that kills most swallowed pathogens.produces acid
- Write the answer. stomach acid
Check. The skin is another barrier. ✓
Method. Read the red curve.
- Read. The second exposure is at 20 at day 4.20
- Write the answer. 20
Check. The first exposure is only 3 at day 4. ✓
Method. Think about the shape.
- Think. An antibody has a shape that fits one specific antigen.fits one specific antigen
- Conclude. So it binds to only that pathogen.only that pathogen
- Write the answer. Accept any sentence that says its shape fits one specific antigen.
Check. Specificity is the key feature. ✓
Method. Recall the toxin neutraliser.
- Recall. Antitoxins neutralise the toxins that some pathogens produce.neutralise the toxins
- Write the answer. antitoxins
Check. Antitoxins act on toxins, not on the pathogen itself. ✓
Total 24 marks. Allow about 24 minutes.
Exercise 13X Exam-style questions
Method. Recall the cause.
- Recall. Measles is caused by a virus.caused by a virus
- Write the answer. virus
Check. Measles is spread by droplets. ✓
Method. Recall the process.
- Recall. Phagocytosis is the engulfing of pathogens.engulfing
- Write the answer. phagocytosis
Check. Phagocytes carry this out. ✓
Method. Recall the molecules.
- Recall. Lymphocytes make antibodies that bind to a specific antigen.bind to a specific antigen
- Write the answer. antibodies
Check. Antitoxins neutralise toxins. ✓
Method. Name two barriers, and how each one works.
- Barrier 1. The skin is a physical barrier that pathogens cannot easily cross.physical barrier
- Barrier 2. Mucus and nose hairs trap dust and microorganisms.trap dust
- Write the answer. Accept any two of: the skin; mucus and nose hairs; stomach acid.
Check. Each barrier works in a different way. ✓
Method. Follow the response step by step.
- Match. A lymphocyte with a matching receptor recognises the antigen.matching
- Clone. The lymphocyte clones itself to make many cells.clones
- Antibodies. The clones make antibodies that bind to the pathogen.make antibodies
- Write the answer. Accept any three of: lymphocytes match the antigen; they clone themselves; they make antibodies; antibodies bind to the pathogen to mark it for destruction.
Check. The order matters. ✓
Method. Link memory cells to protection.
- Memory. Memory cells remain in the blood after the first infection.remain in the blood
- Second. On a second exposure they recognise the antigen quickly and respond faster.respond faster
- Write the answer. Accept any answer that says memory cells remain after the infection, and recognise the antigen quickly on a second exposure.
Check. The second response is faster and bigger. ✓
Method. Describe the virus, the cells and the effect.
- Virus. HIV is a virus.virus
- Cells. It attacks lymphocytes, which fight infection.attacks lymphocytes
- Effect. So the immune system weakens and other infections become hard to fight.hard to fight
- Write the answer. Accept any answer that says HIV attacks lymphocytes, the immune system weakens, and other infections become hard to fight.
Check. The effect is a weakened defence. ✓
Method. Give the cause, the spread and the damage.
- Cause. Malaria is caused by a protist, Plasmodium.protist
- Spread. It is spread by mosquitoes, which act as vectors.vectors
- Damage. Plasmodium damages red blood cells and causes fever.red blood cells
- Write the answer. Accept any three of: a protist causes malaria; it is spread by mosquitoes, which are vectors; it damages red blood cells; it causes fever.
Check. The mosquito is the link. ✓
Method. Divide the peaks, then explain the difference.
- Peaks. The second peak is 30 and the first is 8.30 and 8
- Divide. 30 / 8 = 3.75, so it is about 4 times greater.30 / 8 = 3.75
- Explain. Memory cells from the first exposure make the second response faster and bigger.faster and bigger
- Conclude. So the body is better protected on the second exposure.better protected
- Write the answer. Accept 3.75 times, or about 4 times; and an answer that says memory cells make a faster, bigger response.
Check. 8 x 3.75 = 30. ✓
Method. Describe the barriers, then the cells.
- Barriers. The skin, mucus and stomach acid stop most pathogens entering the body.stop most pathogens
- Phagocytes. Phagocytes engulf and digest any pathogens that get past.engulf and digest
- Lymphocytes. Lymphocytes make antibodies that mark specific pathogens.make antibodies
- Conclude. Together they give layered protection.layered protection
- Write the answer. Accept any answer that describes a barrier that keeps most pathogens out, and white blood cells that engulf or make antibodies against those that get past.
Check. Layers of defence work together. ✓