How does the body keep pathogens out, and how do white blood cells destroy those that get in?
The body's non-specific defence mechanisms, the roles of phagocytes and lymphocytes, antibodies and antigens, antitoxins, and the difference between active and passive immunity.
A focused answer to the WJEC GCSE Biology section 2.8 topic on defence, covering the body's non-specific defences, the roles of phagocytes and lymphocytes, antibodies, antigens and antitoxins, how antibodies destroy pathogens, and the difference between active and passive immunity.
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What this dot point is asking
WJEC wants you to know the body's non-specific defences, the roles of phagocytes and lymphocytes, how antibodies and antigens work, the role of antitoxins, and the difference between active and passive immunity.
Non-specific defences
The body's first defences are non-specific, meaning they act against any pathogen, not a particular one. They try to stop pathogens entering or to remove them quickly.
- Skin forms a physical barrier that pathogens cannot easily cross.
- Mucus in the airways traps pathogens, and cilia (tiny hairs) sweep the mucus away.
- Stomach acid kills many pathogens taken in with food.
- Blood clotting seals cuts so pathogens cannot enter the wound.
White blood cells
If pathogens get past the non-specific defences, white blood cells in the blood defend the body. There are two key types.
Phagocytes
A phagocyte surrounds and engulfs a pathogen, then digests it with enzymes inside the cell. Phagocytes are non-specific: they ingest any pathogen they meet.
Lymphocytes
A lymphocyte produces antibodies and antitoxins.
- Antibodies are specific: each antibody fits only one antigen, like a lock and key. They bind to the antigens on a pathogen and cause the pathogen to clump together or be destroyed.
- Antitoxins neutralise the toxins (poisons) released by some pathogens.
How antibodies destroy pathogens
When a pathogen enters the body, lymphocytes recognise its antigens and start to produce the matching antibody. The antibodies attach to the antigens, marking the pathogens and causing them to clump so they can be destroyed. Because each antibody is specific, the body must make a different antibody for each new pathogen.
Active and passive immunity
There are two ways the body can become immune.
| Type | How antibodies are obtained | Lasting |
|---|---|---|
| Active immunity | The body makes its own antibodies (after infection or vaccination) | Long lasting |
| Passive immunity | The body receives ready-made antibodies (for example from the mother) | Short lived |
- Active immunity is gained when the body makes its own antibodies, for example after catching a disease or after vaccination. It is long lasting because memory cells remain.
- Passive immunity is gained when ready-made antibodies are given to the body, for example a baby receiving antibodies in breast milk. It acts quickly but does not last, because the body has not made its own memory cells.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain how lymphocytes defend the body against a pathogen, using the words antigen and antibody.Show worked answer →
A 4-mark explain question on the immune response.
Each pathogen has specific molecules called antigens on its surface. Lymphocytes (a type of white blood cell) recognise the antigen and produce antibodies that exactly fit it. The antibodies attach to the antigens and cause the pathogens to clump together or be destroyed. Because the antibodies are specific to that antigen, each one only works against one type of pathogen.
Markers reward: antigens on the pathogen; lymphocytes produce specific antibodies; antibodies bind to the antigens; pathogens are destroyed. Saying antibodies "eat" pathogens (that is phagocytes) is a common confusion.
WJEC style3 marksDescribe how a phagocyte destroys a pathogen, and state two non-specific defences that act before this stage.Show worked answer →
A 3-mark question on defence.
A phagocyte is a white blood cell that surrounds and engulfs a pathogen, then digests it with enzymes inside the cell. Before pathogens reach the blood, non-specific defences such as the skin acting as a barrier, mucus and cilia trapping and removing pathogens in the airways, stomach acid killing pathogens in food, or blood clotting sealing wounds, help to keep them out.
Markers reward: phagocyte engulfs and digests the pathogen; plus any two non-specific defences (skin, mucus, cilia, stomach acid, blood clotting). Naming a specific antibody response here loses the non-specific marks.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)