How does the body defend itself against pathogens?
The body's defences against pathogens, including physical and chemical barriers and the role of white blood cells.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on body defences, covering the physical and chemical barriers that keep pathogens out and how white blood cells defend the body by engulfing pathogens and producing antibodies.
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What this dot point is asking
WJEC Double Award Unit 4 wants you to describe the body's defences against pathogens, including physical and chemical barriers and the role of white blood cells.
Keeping pathogens out
These barriers are not specific - they keep out pathogens in general.
White blood cells
If pathogens get past the barriers, white blood cells (part of the immune system) destroy them in two main ways:
- Engulfing pathogens: some white blood cells surround and digest pathogens.
- Producing antibodies: other white blood cells make antibodies that are specific to a particular pathogen, marking it for destruction.
- Producing antitoxins: these neutralise the poisons (toxins) released by some pathogens.
Why the second infection is easier to fight
After an infection, some white blood cells become memory cells that stay in the body. If the same pathogen invades again, these memory cells make the right antibodies much faster and in larger amounts, so the pathogen is destroyed before you feel ill. This is immunity, and it is the reason you usually only catch diseases like chickenpox once. This idea is the basis of how vaccines work.
Why each antibody is specific
A key idea is that antibodies are specific: each type of antibody has a shape that fits one particular pathogen (its antigens), like a lock and key. This is why being immune to one disease (such as measles) does not protect you against a different one (such as flu) - you need a different antibody for each. When a new pathogen invades, the body has to find the white blood cells that make the right antibody, which is why the first infection takes longer to fight than later ones. Understanding that antibodies are specific explains both immunity and why we can catch many different diseases.
The skin and blood clotting
The skin is the body's largest barrier, and it has a clever backup: when it is cut, the blood clots to seal the wound. Platelets in the blood help form the clot, which dries into a scab. The scab plugs the gap in the skin, stopping further blood loss and, importantly, keeping pathogens out while the skin heals underneath. This links the immune defences to the blood you met earlier in the course, and shows how the body's barriers and blood work together to prevent infection.
Try this
Q1. Name the acid in the stomach that kills pathogens. [1 mark]
- Cue. Hydrochloric acid.
Q2. State the two main ways white blood cells destroy pathogens. [2 marks]
- Cue. Engulfing (digesting) them, and producing antibodies.
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 marksDescribe two physical or chemical barriers that stop pathogens entering the body, and explain how each works.Show worked answer →
A Unit 4 describe question worth 4 marks. Reward two of: the skin acts as a physical barrier keeping pathogens out, and clots to seal cuts (1, +1 for how); the stomach produces hydrochloric acid that kills pathogens in food (1, +1); the nose has hairs and mucus that trap pathogens (1, +1); the trachea and bronchi are lined with mucus and cilia that trap and sweep out pathogens (1, +1); and tears contain enzymes that kill bacteria (1, +1). Markers credit two barriers each with how it works. A common error is to name a barrier without explaining how it protects.
WJEC style4 marksExplain two ways white blood cells defend the body against pathogens.Show worked answer →
A Unit 4 explain question worth 4 marks. Reward: white blood cells engulf and digest pathogens (phagocytosis) (2 marks for the idea and how); and white blood cells produce antibodies that lock onto and destroy specific pathogens (2 marks); they can also produce antitoxins to neutralise toxins. Markers credit engulfing pathogens and producing antibodies, each with a brief explanation. A common error is to say white blood cells "eat germs" without the term engulf, or to confuse antibodies with antibiotics.
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