Immunity and medicine

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Big idea: living with pathogens

Big idea (key concept): Relationships. Pathogens and their hosts are in a constant relationship: they evolve and respond to each other. Our immune system, our medicines and our communities are all part of that relationship.
Related concept: Consequences. Choices such as vaccinating, prescribing antibiotics or approving a new medicine have consequences for individuals and for whole populations.
Global context: Globalization and sustainability. Pathogens travel with people and goods around the world. Keeping medicines effective for future generations is a sustainability problem.
A pathogen is a microorganism that causes disease. The four main groups are bacteria, viruses, fungi and protists. A communicable disease is one that can spread from one host to another.
First-line defences stop most pathogens getting in: the skin, mucus and cilia in the airways, lysozyme in tears, and hydrochloric acid in the stomach. If a pathogen gets through, the immune system takes over.

The four groups of pathogen

Inside the body: phagocytes, lymphocytes and antibodies

Every pathogen has molecules on its surface called antigens. The immune system recognises antigens as foreign.
Phagocytes are white blood cells that detect a pathogen, engulf it and digest it with enzymes. This is a non-specific response because it works against many types of pathogen.
Lymphocytes are white blood cells that make a specific response. Each lymphocyte has a receptor that matches one antigen. When it meets that antigen it divides to form a clone of identical cells, and these make large numbers of antibodies.
An antibody has a shape that is complementary to its antigen, so it only fits one type of pathogen. Antibodies bind to pathogens, make them clump together (agglutination), neutralise toxins, and mark the pathogens for destruction by phagocytes. Other lymphocytes, called T cells, destroy body cells that are already infected.
The first time a pathogen is met, the response takes several days, so you may become ill. During that time some lymphocytes become memory cells, which stay in the body for years.
If the same pathogen returns, memory cells respond faster and make more antibodies. This secondary response often stops the pathogen before it makes you ill. That is immunity.

The specific immune response in four steps

Vaccination and herd immunity

A vaccine contains antigens from a pathogen in a harmless form, such as a weakened or inactivated pathogen, or just parts of it. Some vaccines use a molecule of mRNA that tells the body's cells to make a harmless antigen. Whichever type is used, the immune system makes antibodies and memory cells without the person getting the disease. This is active immunity.
A booster dose can strengthen the memory response. Pathogens such as the influenza virus mutate and change their antigens, so memory cells for last year's strain may not recognise this year's. That is why the flu vaccine is updated.
Vaccines can have side effects. Most are mild, such as a sore arm or a short fever. Serious reactions are very rare, and vaccines are monitored after they are approved. Public health experts compare these small risks with the risk from the disease itself.
Herd immunity happens when enough people are immune that a pathogen finds it hard to spread. This helps protect people who cannot be vaccinated, such as babies or people with weakened immune systems. How many people must be immune depends on how easily the disease spreads: for measles, which spreads very easily, about 95% need to be immune.
Passive immunity means receiving ready-made antibodies, for example from a mother through the placenta and breast milk. It protects straight away but only for a short time, because the body has not made its own memory cells.

How a vaccine gives long-term immunity

Antibiotics and resistance

Antibiotics are medicines that kill bacteria or stop them growing. Penicillin, for example, stops bacteria building their cell walls. Water then enters the weakened cell by osmosis and it bursts.
Antibiotics do not work on viruses. Viruses live inside host cells and do not have the cell walls or other structures that antibiotics target.
In a large population of bacteria, a random mutation may make one bacterium resistant to an antibiotic. The antibiotic does not cause the mutation. It simply kills the non-resistant bacteria, so the resistant bacterium survives, reproduces by binary fission, and passes on the resistance gene.
Bacteria can also pass resistance genes on plasmids to other bacteria, even of a different species. Over time, resistant strains such as MRSA become common.
The more an antibiotic is used, the stronger the selection for resistance. Doctors therefore try to prescribe antibiotics only when they are needed, and researchers search for new ones. Resistance is a worldwide problem, because bacteria travel with people.

How antibiotic resistance becomes common

Monoclonal antibodies

Monoclonal antibodies are antibodies that are all identical, because they are made by a single clone of cells. Each one binds to one specific antigen.
To make them, a mouse is injected with the antigen and a lymphocyte that makes the matching antibody is taken. It is fused with a tumour cell, which divides endlessly. The fused cell is called a hybridoma. It divides repeatedly and every cell makes the same antibody, which can be collected and purified.
Uses. In a pregnancy test, monoclonal antibodies bind to the hormone HCG in urine. In diagnosis they can locate a particular molecule. In research they can be given a fluorescent label to show where a molecule is in a tissue. In treatment they can carry a drug to cancer cells by binding to an antigen on their surface.
Limitations. Some patients have side effects. Making them is expensive. Early hopes that they would treat many diseases have only partly been met. Using mice raises ethical questions about animal welfare, which people weigh differently.
Evaluating a technology like this means weighing the potential benefits, such as specific and targeted action, against costs, risks and ethical concerns.

Making a hybridoma cell

Think like a scientist: testing a new medicine

A new medicine is first tested in the laboratory with cells and computer models, and sometimes on animals, which is regulated and debated. Then it goes through clinical trials on people.
Phase 1: a small group, often healthy volunteers, to check safety and a suitable dose. Phase 2: a larger group of patients to see whether it works and to find the best dose. Phase 3: large groups of patients, often thousands, to compare the medicine with the best existing treatment or a placebo.
A placebo looks like the medicine but contains no active ingredient. Comparing with a placebo shows whether any improvement comes from the medicine and not just from expecting to feel better.
To reduce bias, trials are randomised (chance decides who gets which treatment) and often double-blind (neither patients nor doctors know who is receiving which until the end). Results are checked by other scientists (peer review) and by regulators.
Inquiry task 1: in a trial of a new pain medicine, name the independent variable, the dependent variable and two variables that should be controlled. Inquiry task 2: a trial of 20 people finds that 12 improved with the medicine and 9 with the placebo. Evaluate how strongly these data support the medicine.

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1.What is a pathogen?
Easy
- AA cell that makes antibodies
- BA medicine that kills bacteria
- CA microorganism that causes disease
- DA protein found in blood plasma
2.What is an antigen?
Easy
- AA molecule that the immune system recognises as foreign
- BA white blood cell that engulfs bacteria and digests them
- CA protein made by lymphocytes to fit a pathogen
- DA medicine that destroys viruses inside the cells
3.Antibiotics are used to treat infections caused by viruses.
Easy
True or false?
4.Which of these is a first-line defence that helps stop pathogens getting into the body?
Easy
- AAntibodies that are made by lymphocytes in the blood
- BMemory cells that stay in the blood for many years
- CA booster dose of a vaccine given by a nurse
- DMucus that traps microorganisms in the airways
5.Which type of cell makes antibodies?
Easy
- ARed blood cells
- BLymphocytes
- CPhagocytes
- DPlatelets
6.Complete the sentence.
Easy
An antibody has a shape that is ____ to one particular antigen.
7.What is the job of memory cells?
Easy
- ATo engulf and digest any pathogen that they meet
- BTo respond quickly if the same pathogen returns
- CTo carry oxygen from the lungs to the body cells
- DTo make the very first antibodies in a new infection
8.What does a vaccine usually contain?
Easy
- AA harmless form of antigens from a pathogen
- BReady-made antibodies from another person
- CA live and fully active pathogen
- DAn antibiotic that kills bacteria

Plus de questions

9.Antibiotics can be used to treat bacterial infections.
Easy
True or false?
10.Which of these can be pathogens? (select all that apply)
Easy
- ABacteria
- BViruses
- CFungi
- DProtists
- ERed blood cells
11.Match each cell or molecule to its job in the immune system.
Easy
- Phagocyte
- Lymphocyte
- Memory cell
- Antibody
- Binds to one particular antigen
- Engulfs and digests pathogens
- Gives a faster response to a repeat infection
- Makes antibodies when it meets its antigen
12.Put the stages of the specific immune response in order.
Easy
- The clone makes many specific antibodies
- A pathogen with an antigen enters the body
- Antibodies bind to the pathogens so they can be destroyed
- A lymphocyte with a matching receptor recognises the antigen
- The lymphocyte divides to form a clone
13.What is herd immunity?
Easy
- AWhen every single person has had the disease once
- BWhen antibodies are passed to a baby in breast milk
- CWhen enough people are immune that disease spreads poorly
- DWhen a pathogen has become resistant to a medicine
14.What is special about monoclonal antibodies?
Easy
- AThey are all identical and bind to one specific antigen
- BThey are made by many different kinds of lymphocyte
- CThey can kill any pathogen that they meet
- DThey are only found in people with cancer
15.What is a placebo?
Easy
- AA drug that is stronger than the medicine being tested
- BA cell that carries a drug to cancer
- CA group of patients who all get the new medicine
- DA dummy treatment that contains no active ingredient
16.A vaccine works by giving a person the disease so that they become immune.
Easy
True or false?
17.Complete the sentence.
Easy
A pregnancy test uses monoclonal antibodies that bind to the hormone ____ in urine.
18.Which part of the body uses hydrochloric acid to kill many swallowed pathogens?
Easy
- AThe stomach
- BThe skin
- CThe trachea
- DThe tear gland
19.New medicines are tested in clinical trials before they are approved for general use.
Easy
True or false?
20.Complete the sentence.
Easy
Phagocytes are part of the ____ immune response because they attack many types of pathogen.
21.Why is a second infection with the same pathogen usually milder than the first?
Medium
- AThe pathogen has become weaker each time it infects
- BMemory cells respond faster and make more antibodies
- CThe skin builds a much thicker barrier after infection
- DPhagocytes learn how to make antibodies themselves
22.Why does a measles vaccine not protect against flu?
Medium
- AFlu is always caused by bacteria and measles is caused by a virus
- BVaccines only ever work for a single person's whole lifetime
- CAntibodies and memory cells are specific to one pathogen's antigens
- DMemory cells cannot be made as a result of having a vaccine
23.Why does the flu vaccine need to be updated regularly?
Medium
- AAntibodies against flu only survive in the body for a week
- BVaccines make the virus grow stronger in each new year
- CPhagocytes destroy the vaccine before it is able to work
- DThe flu virus mutates, so its antigens change
24.How does herd immunity help a baby who is too young to be vaccinated?
Medium
- AFewer people around the baby can pass on the infection
- BThe baby receives memory cells from every vaccinated person
- CThe baby's skin becomes resistant to the pathogen
- DVaccinated people make antibodies for the baby
25.A baby receives antibodies from its mother in breast milk. Why does this protection last only a short time?
Medium
- ABreast milk contains the weakened pathogen
- BThe baby has not made memory cells, so the antibodies are used up
- CThe baby's lymphocytes destroy the mother's antibodies at once
- DAntibodies from the mother change into antigens
26.Which of these are ways in which antibodies help to defeat pathogens? (select all that apply)
Medium
- AThey bind to pathogens and make them clump together
- BThey mark pathogens so phagocytes can engulf them
- CThey neutralise toxins made by pathogens
- DThey digest pathogens using enzymes
- EThey carry oxygen to infected tissue
27.How does penicillin kill bacteria?
Medium
- AIt makes the bacteria produce antibodies against themselves
- BIt copies the DNA of the bacteria in the wrong way
- CIt stops them building cell walls, so they burst
- DIt starves the bacteria of the oxygen they need to live
28.Which sequence best describes how antibiotic resistance becomes common in a bacterial population?
Medium
- AThe antibiotic makes bacteria mutate, then they all become resistant at once
- BBacteria learn to resist the antibiotic and then pass on what they learned
- CResistant bacteria are made weaker, then replaced by non-resistant bacteria
- DRandom mutation, then non-resistant bacteria are killed, then resistant ones survive and reproduce
29.Taking an antibiotic makes bacteria mutate so that they become resistant.
Medium
True or false?
30.Match each term to its meaning.
Medium
- Active immunity
- Passive immunity
- Primary response
- Secondary response
- The slower first response to a new antigen
- Your body makes its own antibodies and memory cells
- A rapid, large response by memory cells
- Ready-made antibodies given to you
31.Put these events in order to explain how a resistant strain spreads.
Medium
- The resistant bacterium survives and divides
- A random mutation makes one bacterium resistant
- The resistance allele becomes common in the population
- Antibiotic treatment kills the non-resistant bacteria
32.An antibiotic kills 99.9% of the bacteria in a sample of 1 000 000. How many bacteria survive?
Medium
33.How is a hybridoma cell made?
Medium
- AA lymphocyte making a specific antibody is fused with a tumour cell
- BTwo tumour cells from a mouse are fused together in a dish
- CA bacterium is injected into a lymphocyte from a mouse
- DA red blood cell is fused with a phagocyte in a dish
34.What is the main advantage of using monoclonal antibodies to carry a drug to cancer cells?
Medium
- AThe drug can work without the antibody needing any antigen to bind
- BThe drug is concentrated at the cancer cells, so fewer healthy cells are harmed
- CThe antibodies destroy all of the healthy cells in the body as well
- DThe treatment is always cheaper than every other kind of cancer drug
35.Why do clinical trials compare a new medicine with a placebo?
Medium
- ATo make sure that every patient in the trial receives the same dose
- BTo make the trial cheaper and quicker to run than it would be
- CTo separate the effect of the medicine from expecting to feel better
- DTo prove that the medicine works before any data are collected
36.What does double-blind mean in a clinical trial?
Medium
- ANeither patients nor researchers know who has the medicine until the end
- BEvery patient is tested twice, once before and once after treatment
- CTwo different medicines are given to every patient in the trial
- DOnly the researchers know which patients are given the medicine
37.What is the main purpose of a Phase 1 trial?
Medium
- ATo prove the medicine works in thousands of patients
- BTo compare the medicine with all the other medicines in the world
- CTo advertise the medicine to doctors
- DTo check safety and a suitable dose in a small group
38.Larger trials usually give more reliable evidence than very small trials.
Medium
True or false?
39.In a trial comparing a new pain medicine with a placebo, what is the independent variable?
Medium
- AThe level of pain reported by each patient
- BWhich treatment each group receives
- CThe age of the doctors in the trial
- DThe colour of the room
40.Why are viruses not killed by penicillin?
Medium
- AViruses are too large for the penicillin to reach them
- BViruses are always hidden away in the bloodstream
- CViruses have no bacterial cell wall for penicillin to target
- DPenicillin is only able to work during the night
41.
One person (invented data) had a vaccine on day 0 and a second dose on day 28. Their antibody level was measured.
Day Antibody level (arbitrary units)
0 0
7 2
14 15
21 20
28 12
35 90
42 140
Which statement best describes the response to the second dose compared with the first?
Hard
- AIt was faster and reached a much higher level, because memory cells were already present
- BIt was slower and lower, because the antibodies from dose 1 had run out
- CIt was the same as the first response, because memory cells are not involved
- DIt was fast but lower, because the second dose is always a weaker one
- EIt cannot be compared because the doses were given on different days
42.In the same data, the highest level after dose 1 was 20 units (day 21) and the level on day 35 was 90 units. How many times higher is the day 35 level than the highest level after dose 1?
Hard
43.A vaccine trial (invented data) had 10 000 people in the vaccine group and 10 000 in the placebo group. 20 people in the vaccine group and 200 in the placebo group became infected. By what percentage did the vaccine reduce the infection rate compared with the placebo group?
Hard
44.Someone says that vaccines do not work because a vaccinated person caught the disease. What is the best response?
Hard
- AThat one case proves that vaccines never work for anybody at all
- BVaccines lower the chance and severity of illness but are not 100% protective
- CVaccinated people can never catch the disease at any later time
- DRates of disease in vaccinated and unvaccinated groups are not relevant
- EA vaccine only works if the person has already had the disease before
45.In a fully susceptible population, each infected person would infect 4 others. The share of people who must be immune to stop the disease spreading is 1 - 1/R, where R is the number each case would infect. What percentage must be immune?
Hard
46.Hospital A uses antibiotic X very widely. Hospital B uses it only when tests show it is needed. Resistant bacteria are more common in Hospital A. What is the best explanation?
Hard
- AHospital A bacteria mutate on purpose because of the antibiotic use
- BHospital B bacteria learn to avoid antibiotic X by avoiding it
- CWide use selects for resistant bacteria, because others are killed more often
- DAntibiotics make all of the bacteria in a hospital stronger over time
- EResistance in hospitals is only caused by viruses in the hospital
47.Which of these make a clinical trial more reliable? (select all that apply)
Hard
- AChance decides which patients get which treatment
- BNeither patients nor researchers know who has the medicine
- CA large number of patients is used
- DPatients choose which group to join
- EOnly patients who are keen on the medicine take part
48.Explain why a second infection with the same pathogen is usually less severe than the first.
Hard
49.Describe how a vaccine gives long-term protection against a disease.
Hard
50.Explain why antibiotic-resistant bacteria are becoming more common.
Hard
51.Describe how a clinical trial can be designed to give reliable evidence about a new medicine.
Hard
52.Evaluate the use of monoclonal antibodies to treat cancer. Give one benefit and one limitation.
Hard
53.Antibiotics will kill a virus if a high enough dose is given.
Hard
True or false?
54.A trial of 20 people (invented data) found that 12 improved with a new medicine and 9 improved with a placebo. How strongly do these data support the medicine?
Hard
- AStrongly, because more people improved with the medicine than without
- BThey prove that the medicine does not work for any patient
- CThey prove that the placebo is better than any kind of medicine
- DWeakly, because the sample is small and the difference is a few people
- EThey cannot be used because nobody improved while taking a placebo

Notes de révision

## The big idea - Pathogens and hosts are in a constant **relationship**. Medicines and vaccines have **consequences** for people and populations (**Globalization and sustainability**). ## Defences - First line: skin, mucus and cilia, lysozyme, stomach acid. - **Phagocytes** engulf any pathogen (non-specific). - **Lymphocytes** recognise one **antigen**, clone, and make **antibodies** (specific). **Memory cells** give a fast secondary response. ## Vaccination - Harmless antigens give **active immunity** and memory cells. Boosters strengthen it. Flu vaccines change because the virus mutates. - **Herd immunity** protects those who cannot be vaccinated. **Passive immunity** is short-lived because there are no memory cells. - Side effects are usually mild; serious ones are very rare. ## Antibiotics - Kill bacteria, not viruses. Resistance arises by random mutation and is **selected** by antibiotic use. Plasmids can spread it. ## Monoclonal antibodies - Identical, specific antibodies from **hybridoma** cells. Uses: pregnancy tests, research, targeted drugs. Limits: cost, side effects, ethics. ## Testing medicines - Phases 1 to 3, **placebo**, **randomised**, **double-blind**, large samples, peer review.

Pathogen: A microorganism that causes disease.
Communicable disease: A disease that can spread from one host to another.
Antigen: A molecule, usually on the surface of a pathogen, that the immune system recognises as foreign.
Phagocyte: A white blood cell that engulfs and digests pathogens.
Lymphocyte: A white blood cell that makes a specific response, producing antibodies when it meets its antigen.
Antibody: A protein with a shape complementary to one antigen, made by lymphocytes.
Memory cell: A lymphocyte that stays in the body and gives a fast response to a repeat infection.
Vaccine: A harmless form of a pathogen's antigens that gives immunity without causing the disease.
Herd immunity: Protection of a population when enough people are immune that a disease cannot spread easily.
Passive immunity: Short-term protection from ready-made antibodies, with no memory cells.
Antibiotic: A medicine that kills bacteria or stops them growing.
Antibiotic resistance: When bacteria survive an antibiotic because of a gene that was selected by its use.
Monoclonal antibody: An antibody made by a single clone of cells, all identical and specific to one antigen.
Hybridoma: A cell made by fusing a lymphocyte with a tumour cell that divides repeatedly and makes one antibody.
Placebo: A dummy treatment with no active ingredient, used for comparison in a trial.
Double-blind trial: A trial in which neither patients nor researchers know who receives the medicine until the end.

Cartes mémo

What is a pathogen?

A microorganism that causes disease.

Name the four groups of pathogen.

Bacteria, viruses, fungi and protists.

Name three first-line defences.

Skin, mucus and cilia, stomach acid (also lysozyme in tears).

What is an antigen?

A molecule on a pathogen or cell that the immune system recognises.

What does a phagocyte do?

Engulfs and digests pathogens. It is a non-specific response.

What does a lymphocyte do?

Recognises one antigen, divides to form a clone, and makes specific antibodies.

Why are antibodies specific?

Each has a shape complementary to one antigen.

How do antibodies help?

They bind and clump pathogens, mark them for phagocytes and neutralise toxins.

What are memory cells?

Lymphocytes that stay in the body and give a faster, larger response if the pathogen returns.

What does a vaccine contain?

A harmless form of antigens, such as weakened or inactivated pathogens or parts of them.

Why does the flu vaccine change?

The flu virus mutates, so its antigens change and old memory cells may not match.

What is herd immunity?

Protection when enough people are immune that a disease cannot spread easily.

What is passive immunity?

Short-term protection from ready-made antibodies, with no memory cells.

How does penicillin work?

It stops bacteria building cell walls, so they burst.

Why do antibiotics not work on viruses?

Viruses are not cells and have no cell wall or other target.

How does antibiotic resistance spread?

A random mutation makes a bacterium resistant, the antibiotic selects it, and it reproduces. Plasmids can pass the gene on.

How are monoclonal antibodies made?

A lymphocyte is fused with a tumour cell to make a hybridoma that divides and makes one antibody.

Give two uses of monoclonal antibodies.

Pregnancy tests (binding HCG) and targeted drug delivery to cancer cells.

What is a placebo and why is it used?

A dummy treatment, used to separate the effect of the medicine from expecting to feel better.

What makes a clinical trial reliable?

Large samples, random allocation, a comparison group, double-blinding and peer review.

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Day	Antibody level (arbitrary units)
0	0
7	2
14	15
21	20
28	12
35	90
42	140