AS 1 QWC extended response, immunity · 6 marks
Describe how the body produces a specific immune response following infection by a pathogen.
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AS · AS 1 Molecules and Cells
The immune response: cells, antibodies and immunological memory
The short answer
A pathogen's antigens are displayed by phagocytes after phagocytosis. Specific helper T cells bind, activating B cells with complementary receptors. Those B cells divide into plasma cells that secrete antibodies and memory cells that remain, giving a faster, larger secondary response on re-infection.
The sequence for a 6-mark answer
- Phagocytosis: a phagocyte engulfs the pathogen into a phagosome, lysosomes fuse and hydrolytic enzymes digest it.
- Antigen presentation: antigens are displayed on the phagocyte's surface membrane.
- Cell-mediated response: a helper T cell with a complementary receptor binds and is activated, dividing by mitosis to form clones, including cytotoxic T cells.
- Humoral response: helper T cells stimulate a specific B cell, which divides to form clones (clonal selection and expansion).
- Plasma cells secrete large quantities of a specific antibody that binds to the antigen, causing agglutination and marking pathogens for phagocytosis.
- Memory cells persist, so a second exposure triggers a faster, larger antibody response — often before symptoms appear.
Antibody structure
An antibody is a quaternary-structure protein: four polypeptide chains, two heavy and two light, joined by disulfide bonds. The variable region differs between antibodies and forms an antigen-binding site with a shape complementary to one specific antigen; the constant region is the same across antibodies of a class and allows binding to phagocytes.
Types of immunity
| Type | How acquired | Memory cells? |
|---|---|---|
| Active natural | Infection | Yes |
| Active artificial | Vaccination | Yes |
| Passive natural | Antibodies across placenta or in breast milk | No |
| Passive artificial | Injected antibodies (antiserum) | No |
Phrases that earn the marks
- antigen is presented on the phagocyte's surface membrane
- complementary receptor / specific to the antigen
- clonal selection and clonal expansion by mitosis
- plasma cells secrete antibodies
- memory cells give a faster, larger secondary response
- antibodies cause agglutination of pathogens
Where marks get lost
- Saying antibodies 'kill' pathogens; they bind and agglutinate them so phagocytes destroy them.
- Confusing antigen with antibody in a rushed answer.
- Leaving out memory cells when explaining vaccination.
- Using 'the body fights the disease' language instead of named cells.
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Exam questions on immune response a level biology, marked
Two AS-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
AS 1 QWC extended response, immunity · 6 marks
Describe how the body produces a specific immune response following infection by a pathogen.
Command word: Describe — how to answer itMark scheme — 6 creditworthy points
- Phagocyte engulfs the pathogen and presents its antigens on its surface — antigen-presenting cell
- A T helper cell with a complementary receptor binds to the presented antigen
- The T helper cell releases cytokines and activates a specific B lymphocyte — clonal selection
- The B lymphocyte divides by mitosis to form clones — clonal expansion
- Plasma cells secrete large numbers of specific antibodies complementary to the antigen
- Memory cells remain, giving a faster and larger secondary response on re-infection
Full-mark answer
A phagocyte engulfs the pathogen by phagocytosis and displays the pathogen's antigens on its own cell surface membrane, becoming an antigen-presenting cell. A T helper cell with a receptor complementary to that antigen binds to it and releases cytokines, which activate the specific B lymphocyte that also has a complementary receptor — clonal selection. That B lymphocyte divides repeatedly by mitosis to form a clone. Most of these differentiate into plasma cells, which secrete large quantities of a specific antibody that is complementary to the antigen and causes agglutination of the pathogen. Some become memory cells, which remain in the blood so that on re-infection with the same pathogen antibodies are produced faster and in greater quantity — the secondary response.
Examiner insight: Use 'complementary', never 'identical' or 'the same shape'. Clonal selection and clonal expansion are two separate marks, and the secondary response mark needs both 'faster' and 'greater quantity'.
AS 1 application question, antigenic variability · 4 marks
Explain why vaccination against influenza is needed each year, whereas a single course of vaccination against measles gives long-lasting protection.
Command word: Explain — how to answer itMark scheme — 4 creditworthy points
- The influenza virus mutates frequently, changing its surface antigens — antigenic variability
- Existing memory cells and antibodies are no longer complementary to the new antigens
- So there is no secondary response to the new strain and a new vaccine is required each year
- The measles virus shows little antigenic variation, so the memory cells produced remain complementary and give long-lasting immunity
Full-mark answer
The influenza virus mutates frequently, which changes the shape of its surface antigens — antigenic variability. Memory cells and antibodies produced against last year's strain are no longer complementary to the new antigens, so they cannot bind and no rapid secondary response occurs. A new vaccine matched to the circulating strains is therefore needed each year. The measles virus shows very little antigenic variation, so the memory cells formed after vaccination remain complementary to its antigens and continue to give a rapid secondary response for many years.
Examiner insight: The measles half of the comparison is worth a mark on its own; answers that only discuss influenza cap at 3.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- What is the difference between the cell-mediated and humoral responses?
- The cell-mediated response involves T cells acting on infected body cells; the humoral response involves B cells producing soluble antibodies against antigens in body fluids.
- Why is the secondary immune response faster?
- Memory cells from the first exposure are already present in large numbers and divide rapidly into plasma cells, so antibody concentration rises sooner and higher.
- Why does vaccination sometimes need a booster?
- Memory cell numbers fall over time, and a booster raises them again so protection is maintained.
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