Biology · Unit 13

The Immune System

How the body defends itself

The immune system has to solve a genuinely hard problem: recognise anything that should not be there, including things it has never encountered, without attacking the body it lives in.

The unit separates the innate defences - barriers and general responses that work the same way every time - from adaptive immunity, which learns. Antibodies, antigens, vaccination and what happens when the system misfires into allergy or autoimmunity all follow from that split.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-3

What this unit covers

Innate Defences
Adaptive Immunity
Antibodies and Antigens
Vaccines and Immunity
Allergies and Immune Disorders

Where this fits

Overlaps deliberately with Microbes and Disease; take either first. Both are easier after Human Body Systems.

Where people slip

Antigen and antibody sound alike and do opposite jobs. The antigen is the thing being recognised; the antibody is what the body makes in order to recognise it.

How the unit is structured

The Immune System runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Adaptive Immunity

Fill the blankLevel 1
1. After an infection, ____ cells stay in the body so you can respond faster if the same pathogen returns.
- memorycorrect
- phagocyte
- red blood
- stem
Memory cells remain in the body and allow a faster, stronger response on a second exposure.
Multiple choiceLevel 1
2. Which type of white blood cell produces antibodies?
- Lymphocytescorrect
- Red blood cells
- Platelets
- Nerve cells
Lymphocytes are white blood cells that produce antibodies to target specific pathogens.
Fact or fibLevel 2
3. The second time your body meets the same pathogen, the immune response is usually faster and stronger.
Answer: True
Thanks to memory cells, the secondary immune response is quicker and produces more antibodies than the first.
Build the sentenceLevel 3
4. Build a sentence that describes clonal selection.
Answer: An antigen selects a lymphocyte with a complementary receptor
In clonal selection an antigen 'selects' the one lymphocyte whose receptor is complementary to it, and only that cell is then activated.
Choose all that applyLevel 3
5. Which statements correctly describe the cell-mediated immune response? (Select all that apply)
- It involves T lymphocytescorrect
- It is especially important against virus-infected and cancerous cellscorrect
- Its main effector molecules are free-floating antibodies
- Cytotoxic T cells destroy infected body cellscorrect
Cell-mediated immunity uses T lymphocytes, is vital against virus-infected and cancerous cells, and includes cytotoxic T cells that destroy infected body cells; free antibodies belong to the humoral response.
Match the pairsLevel 3
6. Match each immune cell to its main function.
Answer: Plasma cell = Secretes large amounts of antibody; Memory B cell = Gives a rapid response on re-infection; Helper T cell = Releases cytokines to activate other cells; Cytotoxic T cell = Kills virus-infected body cells
Plasma cells secrete antibody, memory B cells give rapid second responses, helper T cells release cytokines, and cytotoxic T cells kill infected cells.

Allergies and Immune Disorders

Fill the blankLevel 1
7. A normally harmless substance that triggers an allergic reaction is called an ____.
- allergencorrect
- antibody
- antibiotic
- enzyme
An allergen, such as pollen or peanuts, is a harmless substance that triggers an allergic reaction in some people.
Multiple choiceLevel 1
8. What causes an allergic reaction?
- The immune system overreacting to a normally harmless substancecorrect
- A total lack of white blood cells
- Too much stomach acid
- A vitamin deficiency only
Allergies happen when the immune system treats a harmless substance, called an allergen, as a threat and overreacts.
Build the sentenceLevel 3
9. Build the sentence describing what goes wrong in a mismatched blood transfusion.
Answer: The recipient's antibodies clump the donor red cells
In a mismatched transfusion the recipient's antibodies bind and clump the donor's red blood cells, a dangerous Type II reaction.
Fact or fibLevel 3
10. HIV is a retrovirus that uses the enzyme reverse transcriptase to copy its RNA into DNA.
Answer: True
As a retrovirus, HIV carries RNA and uses reverse transcriptase to make a DNA copy that is then inserted into the host cell's genome.
Guess the numberLevel 3
11. Roughly what CD4 helper T-cell count, in cells per microlitre of blood, marks the point where HIV infection is classed as AIDS?
Answer: 200 cells per microlitre
AIDS is usually diagnosed when the CD4 T-cell count falls below about 200 cells per microlitre, or when an AIDS-defining infection appears.
Match the pairsLevel 3
12. Match each autoimmune disease to the tissue the immune system attacks.
Answer: Rheumatoid arthritis = Joints; Type 1 diabetes = Insulin-producing cells of the pancreas; Multiple sclerosis = Myelin around nerve cells; Lupus = Many tissues including skin and kidneys
Autoimmune diseases target different self tissues: joints in rheumatoid arthritis, pancreatic cells in type 1 diabetes, myelin in MS, and many tissues in lupus.

Antibodies and Antigens

Fill the blankLevel 1
13. Proteins made by lymphocytes that lock onto antigens are called ____.
- antibodiescorrect
- antigens
- enzymes
- hormones
Antibodies are proteins that bind to specific antigens, helping to destroy pathogens.
Build the sentenceLevel 2
14. Build the sentence describing how antibodies match pathogens.
Answer: Antibodies are specific to one antigen
Each antibody is specific to a single antigen.
Guess the numberLevel 2
15. How many polypeptide chains make up a single antibody molecule?
Answer: 4 chains
An antibody is built from four polypeptide chains: two identical heavy chains and two identical light chains.
Match the pairsLevel 2
16. Match each immune term to its meaning.
Answer: Antigen = A marker on a pathogen's surface; Antibody = A protein that binds one specific antigen; Phagocyte = A cell that engulfs pathogens; Memory cell = Gives a fast response on re-infection
Antigens are markers, antibodies bind them, phagocytes engulf pathogens, and memory cells give a fast future response.
Multiple choiceLevel 2
17. What is an antigen?
- A marker molecule on a pathogen's surface that the immune system recognisescorrect
- A protein that kills bacteria directly
- A type of white blood cell
- A sugar that gives cells energy
Antigens are molecules, often on a pathogen's surface, that the immune system recognises as foreign.
Odd one outLevel 3
18. Which of these is NOT a genuine way that antibodies help destroy pathogens?
- Digesting the pathogen using enzymes in the antibodycorrect
- Agglutinating pathogens into clumps
- Neutralising toxins
- Acting as opsonins to mark pathogens
Antibodies work by agglutination, neutralisation and opsonisation, but they have no digestive enzymes, so they do not digest pathogens themselves.

Innate Defences

Fact or fibLevel 1
19. Mucus in your nose and airways helps trap pathogens before they reach your lungs.
Answer: True
Sticky mucus traps pathogens and dust, and tiny hairs called cilia sweep it away.
Multiple choiceLevel 1
20. What is the body's first line of defence against pathogens?
- The skin, which acts as a physical barrier
- The stomach making insulin
- Memory cells in the blood
- Antibiotics made by the body
The skin is the body's first line of defence, forming a physical barrier that stops most pathogens getting in.
Choose all that applyLevel 2
21. Select all the common signs of inflammation when tissue is infected or injured.
- Rednesscorrect
- Swellingcorrect
- Heat or warmthcorrect
- Hair falling out
Inflammation typically causes redness, swelling and heat as blood flow to the area increases.
Put in orderLevel 2
22. Put the steps of phagocytosis in the correct order.
Answer: A phagocyte detects the pathogen -> The phagocyte surrounds and engulfs the pathogen -> The pathogen is trapped inside a vesicle -> Enzymes digest and destroy the pathogen
A phagocyte detects a pathogen, engulfs it into a vesicle, then digests it with enzymes.
Sequence recallLevel 2
23. Memorise and repeat the order of events in the inflammatory response to a wound.
Answer: Damaged cells and mast cells release histamine -> Blood vessels widen and become leaky -> Fluid and phagocytes move into the tissue -> Phagocytes engulf pathogens and the area heals
Inflammation begins with histamine release, then vasodilation and increased permeability, bringing phagocytes that clear pathogens so healing can occur.
Fill the blankLevel 3
24. The complement fragment C5a attracts phagocytes towards an infection. This movement of cells along a chemical signal is called ____.
- chemotaxiscorrect
- phagocytosis
- osmosis
- agglutination
Chemotaxis is directed cell movement along a chemical gradient; C5a is a chemoattractant that recruits phagocytes to the site of infection.

Vaccines and Immunity

Multiple choiceLevel 1
25. What does a vaccine usually contain?
- A weakened, dead, or harmless part of a pathogen
- A strong dose of live, dangerous pathogens
- Antibiotics
- Red blood cells
Vaccines contain weakened or dead pathogens, or their antigens, so the immune system can learn to fight them safely.
Picture questionLevel 1
26. 💉 What does this medical tool most often deliver to help prevent infectious disease?
- A vaccinecorrect
- An antibiotic cream
- A vitamin gummy
- A sticking plaster
A vaccine is usually injected to train the immune system before you meet the real pathogen.
Fill the blankLevel 2
27. A ____ dose is an extra vaccine given later to top up your immunity.
- boostercorrect
- passive
- primary
- double
A booster dose re-exposes the immune system to the antigen, keeping immunity strong over time.
Guess the numberLevel 2
28. In roughly which year did Edward Jenner create the first successful vaccine, against smallpox?
Answer: 1796 AD
Edward Jenner developed the first vaccine in 1796, using cowpox to protect against smallpox.
Match the pairsLevel 2
29. Match each type of vaccine to what it contains.
Answer: Live attenuated = A weakened but living form of the pathogen; Inactivated = A killed form of the pathogen; Subunit = Just a piece of the pathogen, such as a surface protein; mRNA = Genetic instructions telling your cells to make a viral protein
Vaccines differ in what they contain: weakened live pathogen, killed pathogen, a protein subunit, or mRNA that your cells translate into a viral protein.
Odd one outLevel 2
30. Three of these are types of vaccine. Which one is the odd one out?
- Antihistaminecorrect
- Live attenuated
- Subunit
- mRNA
Live attenuated, subunit and mRNA are all vaccine types; an antihistamine is an allergy medicine, not a vaccine.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise The Immune System

120 questions across 20 steps. Start with step one and crawl at your own pace.

Play this unit

Read about The Immune System

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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