Edexcel IGCSE Biology · Spec 2.62-2.64B
The Immune System
How phagocytes and lymphocytes fight pathogens, why antibodies are specific, and how vaccines give immunity.
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The Immune System
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Explained
Two kinds of white blood cell, and why the second one remembers
When a pathogen gets past the skin and the mucus, white blood cells deal with it. There are two types and they work in completely different ways, so the first thing to fix is which is which.
Phagocytes
A phagocyte engulfs the pathogen, surrounding it and taking it inside the cell. Enzymes then digest it.
This is non specific: the same phagocyte will engulf any pathogen it meets. It is fast, but it leaves no lasting protection.
Lymphocytes and antibodies
A lymphocyte produces antibodies, which are proteins that bind to the pathogen.
Every pathogen carries antigens on its surface, which are the markers that identify it. An antibody has a shape complementary to one particular antigen, so it fits that pathogen and no other. This is what specific means here, and it is why having had one disease gives you no protection against a different one.
The antibodies attach to the antigens, clumping the pathogens together so phagocytes can engulf them more easily, and neutralising the toxins the pathogens release.
Antigen and antibody
The antigen is on the pathogen. The antibody is made by you. The words are similar enough that swapping them is common, and it changes the meaning completely, so it is worth building a habit: antigen is the marker, antibody is the response.
Memory cells and immunity
The first time you meet a pathogen, the lymphocytes take days to produce enough of the right antibody, which is why you feel ill.
Afterwards, some of those lymphocytes remain as memory cells. If the same pathogen returns, they recognise its antigens and produce the correct antibody much faster and in much greater quantity, so it is destroyed before you have any symptoms. That is immunity.
A vaccine gives you that memory without the illness. It contains dead or weakened pathogens, or just their antigens, which cannot cause the disease but do carry the markers. Lymphocytes respond, memory cells form, and you are protected.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks how white blood cells defend the body, for four marks, and lists the points separately: phagocytes engulf pathogens, phagocytes digest or break down pathogens, lymphocytes produce antibodies, antibodies are specific or complementary to the pathogen or its antigens, antibodies attach to the pathogens, and antibodies cause them to clump together.
Engulfing and digesting are two marks, not one. So are producing the antibody and the antibody being specific. Compressing them into a single sentence loses half the available credit.
Elsewhere the same mark scheme, on a question about pests, instructs the examiner to ignore immune. The examiner report on that paper explains why, describing a candidate who had confused the development of resistance with immunity, and advising candidates to make sure they understand the difference.
The distinction is worth holding. Immunity is an individual having memory cells against a pathogen it has met. Resistance is a population of pathogens or pests evolving so that a drug or chemical no longer affects them. One happens within a body, the other across generations.
Active and passive immunity
Active immunity is when your own lymphocytes make the antibodies, either after an infection or after a vaccine. It is slow to develop and long lasting, because memory cells are produced.
Passive immunity is when antibodies are given to you ready made, as a baby receives them through the placenta and in breast milk. It is immediate but temporary, because no memory cells are made and the antibodies are eventually broken down.
Spec 2.62-2.64B
What you need to know
- Describe how phagocytes destroy pathogens
- Describe how lymphocytes make antibodies
- Explain immunity and how vaccines work
Active recall
Quick check
Answer each question before opening the answer.
How do phagocytes and lymphocytes protect the body?
Phagocytes engulf and digest pathogens; lymphocytes produce antibodies against specific antigens.
How does a vaccine give immunity?
It contains a dead/weakened pathogen (antigen); the body makes antibodies and memory cells, so it responds quickly if infected later.
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