Form 4 · Common mistakes

Immunity in Humans, common mistakes

The mistakes SPM students make on Immunity in Humans, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying antibodies destroy pathogens directly by eating them.	It is phagocytes that engulf and digest pathogens; antibodies only bind to antigens and label or disable the pathogen.	State that antibodies agglutinate, neutralise or coat pathogens, and that phagocytes then engulf and destroy them.
Calling passive immunity long-lasting.	No memory cells are formed in passive immunity, so protection fades once the borrowed antibodies break down.	State that passive immunity is immediate but short-lived because no memory cells are formed.
Writing that a vaccine contains antibodies.	A vaccine contains an antigen so the body makes its own antibodies; supplying ready-made antibodies would be passive immunity instead.	State that a vaccine contains a weakened, dead or partial pathogen (the antigen) that triggers the body to make its own antibodies and memory cells.
Confusing antigen and antibody.	They are opposite sides of the response, so swapping them reverses the meaning of the answer.	State that an antigen is the foreign marker on the invader and an antibody is the matching protein the host produces against it.
Believing a vaccine can cause the disease it protects against.	The antigen in a vaccine is weakened, dead or only a fragment, so it cannot cause the full disease.	State that the weakened antigen only triggers antibody and memory cell production, and that any mild symptoms come from the immune response, not the disease.
Treating an allergy as an infection.	An allergen is harmless and is not a living, multiplying pathogen, so the body is overreacting rather than defending itself.	State that an allergy is an exaggerated immune response to a harmless allergen such as pollen or dust.
Assuming every transplanted organ is automatically rejected.	Close tissue matching and immunosuppressant drugs can lower the risk of rejection considerably.	State that rejection happens because the immune system may recognise donor antigens as foreign, and that tissue matching and immunosuppressant drugs reduce this risk.
Treating HIV and AIDS as the same term.	HIV is the virus, while AIDS is the later stage when the immune system is too weakened to resist infections.	State that HIV is the virus that destroys T-lymphocytes and AIDS is the advanced stage of a severely weakened immune system.
Describing the first and second lines of defence as specific.	Only the third line, involving lymphocytes and antibodies, targets a particular antigen; the first two act on anything.	State that the barrier and phagocytes are non-specific, while the lymphocyte response is specific to one antigen.
Saying phagocytes produce antibodies.	Antibodies are produced by lymphocytes; phagocytes engulf and digest pathogens but do not make antibodies.	State that lymphocytes produce antibodies and memory cells, while phagocytes carry out phagocytosis.
Writing that antibodies made against one pathogen protect against all pathogens.	Each antibody has a binding site shaped for one specific antigen, so it does not fit an unrelated antigen.	State that each antibody is specific to one antigen, so it gives no protection against a different, unrelated pathogen.
Saying memory cells produce the symptoms of a disease.	Memory cells simply remain ready; they speed up antibody production in a later infection rather than causing symptoms.	State that memory cells allow a faster, larger secondary response so the pathogen is destroyed before it causes illness.

How to avoid these mistakes

Learn the three lines of defence with one concrete example each.
Compare active and passive, natural and artificial immunity in a single table.
Explain step by step how vaccination gives long-term immunity, from antigen injection to memory cells.
Learn one example each of an allergy, an autoimmune disease and immunodeficiency, and state what goes wrong with the immune response in each.
Practise writing why a transplanted organ may be rejected and how tissue matching or immunosuppressant drugs lower that risk.
Sketch an antibody and link its shape to how it binds one specific antigen.

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.