✋ Skin
The skin is a barrier. It covers the outside of your body, so most pathogens cannot get through it.
Communicable Diseases » Human Defence Systems
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.3.1.6
- The non-specific defences that stop pathogens getting in: skin, nose, trachea and bronchi, and stomach
- What the immune system does if a pathogen gets into the body
- How white blood cells defend you by phagocytosis, antibody production and antitoxin production
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Unlock This CourseKeeping pathogens out
You are surrounded by pathogens every day, yet you are not ill most of the time. This is because your body has defence systems. You have just learned how pathogens spread and how some of them make you ill. Now you need to know how your body fights back.
Some defences are non-specific. They work against all pathogens in the same way, and they try to stop pathogens getting in at all. If a pathogen does get in, the immune system takes over.
Key terms:
- Non-specific defence: a defence that works against all types of pathogen in the same way.
- Immune system: the body's system that tries to destroy any pathogen that enters the body.
- Mucus: a sticky liquid that traps particles and pathogens.
- Cilia: tiny hairs on the surface of some cells that move in a wave-like way.
- Phagocytosis: when a white blood cell engulfs a pathogen and digests it.
- Antibody: a protein made by white blood cells that targets one particular type of pathogen.
- Antitoxin: a protein made by white blood cells that neutralises the toxins made by pathogens.
The four non-specific defences
👃 Nose
The nose has hairs and mucus. The hairs and the sticky mucus trap dust and pathogens in the air you breathe in, before they can travel any further into your body.
🌬 Trachea and bronchi
These airways are lined with mucus and cilia. The mucus traps pathogens and the cilia move the mucus up and away from the lungs, so it can be swallowed or coughed out.
⚗ Stomach
The stomach makes hydrochloric acid. This strong acid kills most of the pathogens that you swallow in your food and drink, and in the mucus from your airways.
These four defences are all non-specific. The acid in the stomach does not care which pathogen it is meeting: it kills them all. Together they guard three ways in: through the skin, through the air you breathe and through your mouth.
Worked example
A student breathes in air containing bacteria on a dusty path. Where might the bacteria be stopped? First, the hairs and mucus in the nose may trap them. If some get past, mucus in the trachea and bronchi traps them and the cilia move the mucus upwards. If the mucus is swallowed, the acid in the stomach kills the bacteria. That is three defences in a row.
The immune system
Sometimes a pathogen gets past the non-specific defences. For example, it might enter through a deep cut or in a mosquito bite. Then the immune system tries to destroy the pathogen.
The white blood cells are the main part of the immune system. You already know that white blood cells are found in the blood. Now you need to know what they do. They defend against pathogens in three ways.
How white blood cells defend you
🍴 Phagocytosis
A white blood cell engulfs the pathogen, takes it inside itself and digests it. The pathogen is destroyed inside the white blood cell.
🛡 Antibodies
White blood cells produce antibodies. An antibody targets a particular pathogen and helps to destroy it. Each antibody works against one type of pathogen.
☠ Antitoxins
White blood cells produce antitoxins. Some pathogens release toxins that make you ill. An antitoxin neutralises the toxin so that it can no longer harm you.
Look at the difference between the three. Phagocytosis and antibodies are aimed at the pathogen itself. Antitoxins are aimed at the toxins that the pathogen makes. A bacterium can cause problems in both ways, so the body needs both.
Putting it together
Think of your body as a castle with two lines of defence. The outer wall is the non-specific defences: skin, nose, trachea and bronchi and stomach. They try to keep every pathogen out. The soldiers inside are the white blood cells of the immune system. They hunt down any pathogen that gets in, by eating it, by making antibodies or by making antitoxins.
Common mistakes
Saying that white blood cells "kill" pathogens without saying how. Marks are given for the method: phagocytosis, antibodies or antitoxins. Another mistake is saying that antibodies neutralise toxins. That is the job of antitoxins. A third mistake is saying that the stomach "digests" pathogens. It is the acid that kills them.
Exam-style question
(a) Name two parts of the body that act as non-specific defences against pathogens. [2 marks]
(b) Describe how stomach acid helps to defend the body. [1 mark]
(c) A pathogen enters the blood. Describe three ways that white blood cells help to defend the body. [3 marks]
Model answer
(a) Any two from: skin (1), nose (1), trachea and bronchi (1), stomach (1).
(b) The acid kills pathogens that have been swallowed (1).
(c) By phagocytosis, engulfing and digesting the pathogen (1); by producing antibodies that target the pathogen (1); by producing antitoxins that neutralise the toxins it releases (1).
Exam tip
In part (c) the question says "three ways", so name each method clearly and say what it does. Do not just write "they fight the pathogen".