Form 4 · Worked answers

Immunity in Humans, worked answers

Fully worked answers for Immunity in Humans, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Comparing active and passive immunity in a table, including how quickly each acts and how long protection lasts.
Explaining how a vaccine gives long-term protection by triggering memory cell formation.
Describing the three lines of defence in the order a pathogen would meet them.
Explaining how allergy, autoimmune disease and immunodeficiency each arise from a different fault in the immune response.
Describing why a transplanted organ may be rejected and how that risk is lowered.
Linking HIV infection to a falling T-lymphocyte count and greater vulnerability to other infections.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Describe the three lines of defence against pathogens, in the order a pathogen would meet them.

[6]

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The first line of defence is a non-specific barrier that stops pathogens entering: the skin forms a physical barrier, mucus traps pathogens in the airways, and stomach acid and the enzyme lysozyme in tears kill many of them. If a pathogen breaks through, it meets the second line, which is also non-specific: phagocytes engulf the pathogen by phagocytosis and digest it with enzymes. The third line is specific: lymphocytes recognise the particular antigen on the pathogen, multiply, and produce antibodies that match only that antigen, while also forming memory cells for a faster response in future.

first line barrier non-specificskin mucus stomach acid lysozymephagocytes engulf phagocytosislymphocytes specificantibodies match antigenmemory cells

Explain the three ways in which antibodies act against pathogens.

[3]

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By agglutination, antibodies clump pathogens together so that a phagocyte can engulf many of them at once. By neutralisation, antibodies bind to the toxins released by pathogens and inactivate them, preventing the toxins from damaging body cells. By coating the pathogen, antibodies make phagocytes recognise and engulf it faster. In all three, the antibody labels or disables the pathogen, and it is the phagocyte that finally destroys it.

agglutination clumpneutralise toxinscoat pathogenphagocyte engulfsantibody labels not eats

Compare active immunity and passive immunity, including how each is gained, how quickly it acts and how long it lasts.

[4]

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In active immunity the body makes its own antibodies, either after a real infection or after vaccination, and forms memory cells in the process, so it is slow to develop but long-lasting. In passive immunity the body receives ready-made antibodies from another source, such as from the mother across the placenta or by injection, and no memory cells are formed, so it acts immediately but is short-lived. Active immunity therefore gives lasting protection, while passive immunity gives instant but temporary protection.

active makes own antibodiesmemory cells formslow but long-lastingpassive ready-made antibodiesimmediate but short-lived

Explain how a vaccine gives a person long-term protection against a disease.

[5]

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A vaccine contains a weakened, dead or partial pathogen that acts as an antigen without causing the disease. The antigen triggers lymphocytes to multiply and produce antibodies against it, and importantly they also form memory cells. If the real pathogen later enters the body, the memory cells recognise its antigen at once and produce antibodies much faster and in larger amounts than during the first exposure. This rapid secondary response destroys the pathogen before it can cause illness, giving long-term protection.

antigen not antibodieslymphocytes produce antibodiesmemory cells formsecondary response faster and largerpathogen destroyed before illness

Distinguish between an antigen and an antibody, and state where each is found.

[3]

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An antigen is a foreign marker, usually a protein, found on the surface of a pathogen, a toxin or transplanted tissue. An antibody is a Y-shaped protein made by lymphocytes, with a binding site shaped to fit one specific antigen. The antigen is found on the invader, while the antibody is produced by the host to match and bind that antigen.

antigen foreign marker on pathogenantibody made by lymphocytesY-shaped proteinbinding site specificantigen on invader antibody from host

Explain why HIV infection makes a person far more likely to suffer serious infections.

[4]

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HIV infects and destroys T-lymphocytes, which normally help to coordinate the immune response and support antibody production. As the number of functioning T-lymphocytes falls, the third line of defence can no longer mount an effective specific response. The body becomes unable to clear pathogens it would normally deal with easily, so infections that are usually mild can become serious or fatal, a stage known as AIDS.

HIV destroys T-lymphocytescoordinate immune responsespecific response failscannot clear pathogensAIDS stage

Explain why a transplanted organ may be rejected and describe two ways the risk of rejection is reduced.

[4]

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A transplanted organ carries antigens that differ from the recipient's own, so the recipient's immune system may recognise the graft as foreign and produce antibodies and lymphocytes that attack it, which is rejection. The risk is reduced by matching the tissue types of donor and recipient as closely as possible, so fewer antigens differ, and by giving immunosuppressant drugs that lower the activity of the immune system. These measures reduce but do not remove the risk, so a close relative is often a better match.

donor antigens differrecognised as foreignimmune system attacks grafttissue matchingimmunosuppressant drugs

Explain the difference between an allergy and an infection.

[3]

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In an infection the immune system responds to a living pathogen that is multiplying in the body, producing antibodies and using phagocytes to destroy it. In an allergy the immune system overreacts to a normally harmless substance called an allergen, such as pollen or dust, treating it as dangerous and releasing chemicals that cause symptoms like sneezing, a rash or swelling. The allergen is not a pathogen and is not multiplying, so an allergy is a faulty, exaggerated response rather than a defence against disease.

infection living pathogenallergy harmless allergenoverreactionchemicals released symptomsnot a pathogen

Phrasing that earns marks

Body defence: The first line of defence, skin, mucus in the airways, and stomach acid, forms a physical and chemical barrier that stops most pathogens from entering the body. The second line is non-specific: phagocytes engulf and digest any pathogen that slips past the barrier. The third line is specific: lymphocytes recognise particular antigens and produce matching antibodies.
Antibodies: Antibodies are Y-shaped proteins made by lymphocytes. Each type has a binding site shaped to match one specific antigen, much like a key fits one lock, so the body builds up a different antibody for every pathogen antigen it has encountered before. This specificity is why an antibody made against one pathogen normally gives no protection against a different, unrelated pathogen.
Actions of antibodies: Antibodies act in three ways: they agglutinate pathogens by clumping them together so phagocytes can engulf many at once, they neutralise toxins that pathogens release, and they coat pathogens so phagocytes recognise and destroy them faster. This combined action clears an infection while limiting the damage a pathogen can do first.
Active immunity: Active immunity develops when the body's own lymphocytes produce antibodies, either after fighting a real infection or after vaccination with a weakened antigen. Because memory cells are formed in the process, this protection takes time to build up but then lasts for years, sometimes for life.
Passive immunity: Passive immunity happens when ready-made antibodies from another source enter the body, such as those a baby receives from its mother through the placenta or breast milk, or antibodies given by injection after exposure to a dangerous pathogen. Because no memory cells are made, protection is immediate but fades within weeks or months.
Vaccination: Vaccination introduces a weakened, dead or partial form of a pathogen, the antigen, into the body without causing the disease itself. Lymphocytes respond by producing antibodies and, importantly, memory cells, so a real infection later triggers a much faster and stronger antibody response than the first exposure did.
Allergy: An allergy is an exaggerated immune response to a normally harmless substance called an allergen, such as pollen, dust mites or certain foods. The immune system treats the allergen as dangerous and releases chemicals that cause symptoms such as sneezing, rashes or swelling; a severe reaction, anaphylaxis, needs urgent treatment.
Autoimmune disease: An autoimmune disease occurs when the immune system fails to distinguish the body's own cells from foreign antigens and attacks healthy tissue as if it were a pathogen. The tissue that is targeted determines the symptoms, for example joint damage when the attack is directed at the lining of the joints. Common examples studied at this level include the immune system attacking the pancreas or the joints, which is why such conditions usually need long-term medical management rather than a single cure.

Frequently asked questions

What is the difference between active and passive immunity?

Active immunity is when the body makes its own antibodies, either after catching a disease or after vaccination. It is slow to develop but long-lasting because memory cells are formed. Passive immunity is when ready-made antibodies are received, for example from the mother across the placenta or in an injection. It works immediately but is short-lived because the body does not make memory cells.

How does a vaccine work?

A vaccine contains a weakened, dead or harmless part of a pathogen, called the antigen. When injected, it triggers lymphocytes to produce antibodies and memory cells without causing the disease. If the real pathogen infects later, the memory cells produce antibodies quickly, giving long-term protection. Booster doses are sometimes needed because the level of memory cells and antibodies from a single dose can fade over time, especially for some pathogens.

What are the three lines of defence?

The first line is a barrier, skin, mucus and stomach acid, that stops pathogens entering. The second line is non-specific: phagocytes engulf any pathogens that get in. The third line is specific: lymphocytes produce antibodies against particular antigens and form memory cells. Together, these three lines work as a layered system, so a pathogen that gets past one still has to overcome the next.