How does the body recognise and respond to pathogens and foreign cells?
Cell recognition by antigens, including self and non-self; the cellular and humoral immune responses involving phagocytes, T lymphocytes and B lymphocytes; the structure and function of antibodies; the primary and secondary responses and immunological memory; active and passive, natural and artificial immunity, vaccines and herd immunity; antigenic variation; and the use of monoclonal antibodies and the ELISA test.
A focused answer to the AQA 3.2 dot point on cell recognition and immunity. Covers antigens, phagocytosis, the cellular and humoral responses, antibody structure, immunological memory, vaccination, monoclonal antibodies and the ELISA test.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA wants you to explain how the immune system recognises self from non-self using antigens, describe the cellular and humoral responses, give the structure and function of antibodies, explain immunological memory and vaccination, and describe monoclonal antibodies and the ELISA test.
The answer
Antigens, self and non-self
Each type of cell has its own antigens. The immune system identifies pathogens, abnormal (cancer) cells, toxins and cells from other individuals (which is why transplanted organs can be rejected).
Phagocytosis (a non-specific response)
A phagocyte (such as a neutrophil or macrophage) engulfs and destroys pathogens:
- The phagocyte is attracted to and binds to antigens on the pathogen.
- It engulfs the pathogen into a vesicle called a phagosome.
- A lysosome fuses with the phagosome and its hydrolytic enzymes (lysozymes) digest the pathogen.
- The phagocyte displays the pathogen's antigens on its surface, becoming an antigen-presenting cell (APC), which activates the specific response.
The cellular response (T lymphocytes)
T lymphocytes mature in the thymus and respond to antigens presented on body cells.
- A specific helper T cell with a complementary receptor binds to the antigen on an APC.
- This activates the helper T cell to divide and release chemical signals (cytokines) that:
- stimulate phagocytes,
- stimulate B lymphocytes to divide,
- activate cytotoxic T cells, which kill virus-infected and abnormal cells by releasing a protein (perforin) that makes holes in their membranes.
The humoral response (B lymphocytes)
B lymphocytes respond to antigens in body fluids.
- Each B cell carries a different antibody. The one with the antibody complementary to the antigen is selected (clonal selection).
- Stimulated by helper T cells, it divides rapidly by mitosis (clonal expansion) into:
- Plasma cells, which secrete large numbers of antibodies into the blood, and
- Memory cells, which remain for long-term immunity.
Antibody structure and function
Antibodies work by:
- Agglutination: each antibody binds two pathogens, clumping them together so phagocytes engulf many at once.
- Neutralising toxins and marking pathogens for phagocytosis.
Primary and secondary responses
The primary response (first exposure) is slow, because the few B cells with the right antibody must be selected and expanded before antibody levels rise. The person may show symptoms. Memory cells are produced.
The secondary response (re-exposure to the same antigen) is faster and stronger, because memory cells divide quickly into plasma cells, producing a higher concentration of antibody sooner, often before symptoms appear. This is immunological memory.
Types of immunity
| Type | How it is gained | Memory cells? |
|---|---|---|
| Natural active | Catching the disease and recovering | Yes |
| Artificial active | Vaccination | Yes |
| Natural passive | Antibodies from mother (placenta or breast milk) | No |
| Artificial passive | Injection of ready-made antibodies (antiserum) | No |
Active immunity means your own cells make the antibodies; it is long-lasting. Passive immunity means antibodies are given to you; it is immediate but temporary.
Vaccination and herd immunity
A vaccine introduces antigens (dead, weakened or fragments of a pathogen) to trigger a primary response and produce memory cells, without causing the disease. Herd immunity occurs when a large enough proportion of the population is immune that the pathogen cannot spread easily, protecting even the unvaccinated.
Antigenic variation is when a pathogen changes its surface antigens (for example the influenza virus). Memory cells from a previous exposure no longer recognise it, so a primary response must start again, which is why flu vaccines are reformulated each year.
Monoclonal antibodies and the ELISA test
Monoclonal antibodies are identical antibodies produced from a single clone of B cells, all specific to one antigen. Uses include:
- Targeted medicine: an antibody specific to a cancer-cell antigen carries a drug directly to the tumour, sparing healthy cells.
- Pregnancy tests: detect the hormone hCG in urine.
- Diagnosis of disease through the ELISA test.
The ELISA test uses antibodies to detect the presence and quantity of a specific antigen (or antibody). An antibody complementary to the target binds it; a second antibody carrying an enzyme is added; a substrate is added that the enzyme converts into a coloured product. The intensity of colour indicates how much antigen is present.
Examples in context
Example 1. Pregnancy test strips. A home pregnancy test uses monoclonal antibodies specific to the hormone hCG. hCG in the urine binds mobile dye-linked antibodies, which then bind a fixed antibody line, producing a coloured band. The specificity of the antibody for hCG means only pregnancy (not other hormones) gives a positive result.
Example 2. Seasonal flu vaccines and antigenic variation. The influenza virus frequently changes its surface antigens (haemagglutinin and neuraminidase). Memory cells from last year's strain no longer recognise the new antigens, so each year health authorities predict the circulating strains and reformulate the vaccine to trigger a fresh primary response against the new antigens.
Try this
Q1. Describe the stages of phagocytosis. [4 marks]
- Cue. Phagocyte binds to antigens on the pathogen; engulfs it into a phagosome; a lysosome fuses and hydrolytic enzymes digest the pathogen; antigens are displayed on the surface (antigen presentation).
Q2. Explain why the secondary immune response is faster than the primary response. [3 marks]
- Cue. Memory cells from the primary response remain; on re-infection they recognise the antigen and divide rapidly into plasma cells, producing more antibody more quickly, often before symptoms appear.
Q3. Explain why a monoclonal antibody binds to only one type of antigen. [2 marks]
- Cue. Its variable region has a specific tertiary structure that is complementary in shape to only one antigen, so it binds only that antigen.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2020 AQA5 marksDescribe how a vaccine leads to long-term protection against a disease.Show worked answer →
A 5-mark answer needs the primary response, memory cells, and the faster secondary response.
- The vaccine contains antigens from the pathogen (dead, weakened or fragments).
- The antigens trigger a primary immune response: a specific B lymphocyte with a complementary antibody is selected and clonally selected and divides (clonal expansion) into plasma cells and memory cells.
- Plasma cells secrete antibodies and the person becomes immune; the symptoms are avoided because the pathogen is inactivated.
- Memory cells remain in the body for years.
- On re-infection with the same pathogen, memory cells trigger a faster and stronger secondary response, producing antibodies quickly so the pathogen is destroyed before symptoms appear.
Markers reward the antigens, the primary response producing memory cells, and the faster, larger secondary response.
2019 AQA2 marksExplain why the production of monoclonal antibodies can be used to target a specific type of cancer cell.Show worked answer →
A 2-mark answer links specificity to complementary binding.
- A monoclonal antibody has a variable region with a specific tertiary structure that is complementary to one antigen only, in this case an antigen on the cancer cell.
- So the antibody binds only to the cancer cells (and not healthy cells), allowing a drug attached to it to be delivered to the cancer cells alone.
Markers reward the complementary, specific binding to the cancer-cell antigen.
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