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You now know what pathogens are and how they spread. This lesson looks at how your body fights back. There are two lines of defence. First, non-specific barriers try to stop pathogens entering at all — your skin, the mucus and cilia in your airways, your stomach acid and your tears. Second, if pathogens do get in, your white blood cells of the immune system attack them — by engulfing them, by producing antibodies and by producing antitoxins — and afterwards leave behind memory cells that give long-term immunity. This is the heart of the health strand of Topic B6 and sets up the next lesson on vaccines.
By the end of this lesson you should be able to describe the body's non-specific defences, explain the three roles of white blood cells (phagocytosis, antibody production and antitoxin production), explain how antibodies recognise pathogens, and describe how memory cells provide immunity.
Before the immune system is ever needed, the body tries to stop pathogens entering in the first place. These non-specific defences work against all pathogens, not just one.
| Defence | How it protects |
|---|---|
| Skin | A tough physical barrier; if cut, it scabs over to reseal. It also produces oils and supports harmless bacteria that crowd out pathogens |
| Mucus and cilia in the airways | Sticky mucus traps pathogens and dust; tiny hairs called cilia sweep the mucus up and out of the lungs to be swallowed or removed |
| Stomach acid | The stomach produces strong hydrochloric acid that kills most pathogens swallowed in food, drink or mucus |
| Tears | Contain enzymes (lysozyme) that kill bacteria on the surface of the eye; they also wash pathogens away |
| Nose | Hairs and mucus trap pathogens in inhaled air |
Exam Tip: These are non-specific defences — they work against any pathogen and are not learned. Don't confuse them with the specific immune response (antibodies), which is targeted at a particular pathogen. A common exam question asks you to name two ways the body stops pathogens entering; mucus-and-cilia and stomach acid are reliable answers.
If a pathogen breaks through the barriers and enters the body, the immune system responds. The key players are the white blood cells, which patrol the blood and tissues. They defend the body in three ways.
Some white blood cells (called phagocytes) destroy pathogens by engulfing them. The phagocyte changes shape to surround the pathogen, takes it inside the cell, and then digests it using enzymes. Because phagocytes will engulf any pathogen, this is a non-specific response.
Other white blood cells (called lymphocytes) make antibodies. Every pathogen carries unique marker molecules on its surface called antigens. A lymphocyte makes an antibody with a shape that is complementary to a specific antigen — they fit together like a lock and key. The antibody locks onto the antigen, marking the pathogen for destruction and causing pathogens to clump together so they are easier to engulf.
Because each antibody fits only one type of antigen, this is a specific response: a particular antibody works against a particular pathogen and no other. This is also why being immune to one disease — say, chickenpox — gives you no protection against a different one such as measles: the antibodies you made have the wrong shape for the new pathogen's antigens, so the slow first response has to happen all over again.
It is worth understanding why the immune system attacks pathogens but leaves your own cells alone. Your own body cells carry antigens too, but the immune system learns to treat these as "self" and does not attack them. A pathogen's antigens are recognised as "non-self" (foreign), which is what triggers the response. This is also the reason transplanted organs can be rejected: the donor's cells carry slightly different antigens, which the recipient's immune system may recognise as non-self and attack — which is why transplant patients take drugs to suppress this response.
Some white blood cells produce antitoxins. These are chemicals that neutralise the toxins released by bacteria, stopping the toxins from damaging the body's cells. Antitoxins deal with the poisons a pathogen makes, rather than the pathogen itself.
A helpful way to keep antibodies and antitoxins apart is to think about what each targets. Recall from the last lesson that some bacteria (such as the one causing food poisoning) make us ill mainly through the toxins they release. Antibodies target the pathogen (by locking onto its antigens), while antitoxins target the toxins the pathogen produces. Both are made by white blood cells, but they tackle different parts of the problem — one the invader itself, the other its poisons.
These three terms look alike and are confused constantly in exams, so it is worth pinning down exactly what each one is, where it comes from and what it does.
| Term | What it is | Made by | What it does |
|---|---|---|---|
| Antigen | A marker molecule on the surface of a pathogen (or any foreign cell) | The pathogen | Acts as the "identity tag" the immune system recognises as foreign |
| Antibody | A Y-shaped protein with a shape complementary to one antigen | The body's white blood cells (lymphocytes) | Locks onto the matching antigen, labelling and clumping pathogens for destruction |
| Antitoxin | A protein that binds to and neutralises a toxin | The body's white blood cells | Stops bacterial toxins damaging the body's cells |
The key relationships are: the antigen is the target and is part of the pathogen; the antibody is the body's answer to that target and fits it lock-and-key; the antitoxin deals not with the pathogen itself but with the poison it releases. Only the antigen comes from the pathogen — the other two are made by you.
Exam Tip: Keep the three white-blood-cell jobs distinct: phagocytosis engulfs and digests pathogens; antibodies lock onto antigens to label and clump pathogens; antitoxins neutralise bacterial toxins. Mixing up antibodies and antitoxins is one of the commonest slips in this topic.
When you meet a new pathogen, it takes a few days for the right lymphocyte to multiply and make enough of the correct antibody — which is why you feel ill at first. But once the infection is defeated, some of those lymphocytes remain as memory cells.
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