Chapter 3 of 8

All 8 chapters
  1. 1Kinds of disease, readunderstood
  2. 2How diseases spread, readunderstood
  3. 3The body's defences, readunderstood
  4. 4Vaccination, readunderstood
  5. 5Antibiotics and medicines, readunderstood
  6. 6Malaria, HIV and tuberculosis, readunderstood
  7. 7Lifestyle and disease, readunderstood
  8. 8Sexual health, readunderstood

S4.4.3 Chapter 3

The body's defences

Germs are everywhere. Why are we not ill all the time?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Describe skin, mucus and stomach acid as defences
  • Explain how white blood cells engulf pathogens and make antibodies

You breathe in pathogens, swallow them and pick them up on your skin every day. Most days you do not become ill. The body has two lines of defence. The first line keeps pathogens out. The second line destroys the ones that get in.

A table with the columns Line, What it does. first, keeps pathogens out. second, kills those inside.LineWhat it doesfirstkeeps pathogens outsecondkills those inside
The two lines of defence.

The first line has three parts. We take them in turn. Look at each picture and find the labels.

The skin is a tough barrier that most pathogens cannot get through. If the skin is cut, the blood clots and a scab seals the cut. This stops pathogens getting in while the skin heals.

A small picture of skin as a hatched band. Three rods above it stand for pathogens. An arrow from each rod points at the skin and ends at a short bar, so none of them get through. The labels are skin and pathogen.pathogenskin
Skin is a barrier. A rod is a pathogen, drawn much too large. An arrow that ends at a bar is a pathogen that cannot pass.

You must breathe, so the airways cannot be closed. Instead, the airways are lined with cells that make mucus, a sticky liquid. Pathogens and dust stick to the mucus. Other cells in the lining have cilia, which are tiny hairs. The cilia beat together and sweep the mucus up and out of the airways to the throat. There it is swallowed or coughed out.

A picture of an airway. A hatched wall has a row of tiny hairs standing up from it. Above them is a band of mucus drawn as dots, with two rods stuck in it. An arrow along the top has the words swept out. The labels are mucus, pathogen, cilia and swept out.swept outmucuspathogencilia
Mucus traps pathogens. Cilia sweep the mucus out of the airway.

Mucus that is swallowed, and pathogens that come in with food and drink, reach the stomach. The stomach makes a strong acid. It kills most of the pathogens that reach it.

A picture of a stomach, drawn as a bag, with dots for the acid. Rods with broken outlines inside stand for pathogens that are killed. The labels are stomach acid and pathogen killed.stomach acidpathogen killed
Stomach acid kills pathogens. A rod with a broken outline is a dead pathogen.

Some pathogens still get in, for example through a cut or in a breath. Then the second line begins. Blood carries white blood cells. They are fewer than red blood cells, but they are the cells that defend the body. They have three jobs, and we take them in turn.

The first job is to take in pathogens. A phagocyte is a white blood cell that reaches out, wraps round a pathogen and takes it inside. Enzymes in the cell then digest the pathogen, which means they break it down. The picture shows the three steps.

Draw it first

A phagocyte takes in a bacterium

  1. Three pictures, one for each step, drawn on top of each other. In the first, a white blood cell with a lobed nucleus reaches out towards a bacterium. In the second, the white blood cell wraps round the bacterium. In the third, the bacterium is inside the cell and is broken into pieces. The labels are white blood cell, bacterium and bacterium digested.white bloodcellbacterium

    1 The white blood cell reaches out towards a bacterium.

  2. Three pictures, one for each step, drawn on top of each other. In the first, a white blood cell with a lobed nucleus reaches out towards a bacterium. In the second, the white blood cell wraps round the bacterium. In the third, the bacterium is inside the cell and is broken into pieces. The labels are white blood cell, bacterium and bacterium digested.white bloodcellbacterium

    2 It wraps round the bacterium.

  3. Three pictures, one for each step, drawn on top of each other. In the first, a white blood cell with a lobed nucleus reaches out towards a bacterium. In the second, the white blood cell wraps round the bacterium. In the third, the bacterium is inside the cell and is broken into pieces. The labels are white blood cell, bacterium and bacterium digested.white bloodcellbacteriumdigested

    3 The bacterium is now inside the cell. The cell digests it, and the pieces are shown broken apart.

The second job is to make antibodies. Every pathogen has chemicals on its surface called antigens. Your body does not make them. White blood cells notice an antigen they do not know and make antibodies against it. An antibody is a protein with a tip of one shape. It fits one kind of antigen, as a key fits one lock.

A pathogen drawn as a dotted rounded block with four triangle shaped bumps on its top. Two Y shaped antibodies sit on it. The tip of each antibody arm is cut to the shape of a triangle, so it fits a bump exactly. The labels are antibody, antigen and pathogen.antibodyantigenpathogen
An antibody fits its antigen. A different antibody, with a different tip, would not fit.

Antibodies stick to the pathogens. This can clump the pathogens together, so that phagocytes can find them. The third job is to make antitoxins. An antitoxin cancels a poison that a pathogen makes.

The body takes days to make enough antibodies against a new pathogen. By then the pathogen may have multiplied, and you may feel ill. After the infection, a few of the white blood cells stay in the blood for years. These are memory cells. They remember the antigen.

If the same pathogen gets in again, the memory cells make the right antibody quickly and in large amounts. The pathogen is destroyed before it can make you ill. You are then immune to that disease. This protection is called immunity. The graph shows the pattern. The numbers are an example.

A line graph with two lines. The x axis is days after exposure, from 0 to 20 in steps of 5. The y axis is antibody level, from 0 to 100 in steps of 20. The solid line is labelled first exposure. Its points are 0 days and 0, 5 days and 5, 10 days and 20, 15 days and 40, 20 days and 30. The dashed line is labelled second exposure. Its points are 0 days and 0, 5 days and 50, 10 days and 80, 15 days and 60, 20 days and 40. The dashed line rises sooner and goes higher.05101520020406080100secondexposurefirstexposuredays after exposureantibody level
Antibody level after a first and a second exposure to the same pathogen. A typical pattern with example numbers, not a measurement.

Worked example

Use the graph. How many times higher is the highest antibody level after the second exposure than after the first?

  1. The solid line peaks at 40. The dashed line peaks at 80.
  2. Divide the second peak by the first: 80 ÷ 40 = 2.

Answer: 2 times

Worked example

Use the graph. How many days sooner does the antibody level peak after the second exposure than after the first?

  1. The solid line peaks on day 15.
  2. The dashed line peaks on day 10.
  3. The second peak comes 15 − 10 = 5 days sooner.

Answer: 5 days

Blood holds far more red cells than white cells. Take 1 mm³ of blood from a healthy adult. A count might find about 5 000 000 red blood cells and 7 000 white blood cells. There are few white cells, but they work all the time.

Try it

A sticky trap for dust

You need: two pieces of card or stiff paper, a few drops of cooking oil, a pinch of flour, a tray.

Be safe: Do not blow the flour or breathe in the dust. Keep it away from your face and do not taste it. Wipe up with a damp cloth. Do not do this if flour makes you ill.

  1. Rub a thin film of oil over one card. Leave the other card dry.
  2. Hold both cards over the tray. Sprinkle a pinch of flour on each.
  3. Tilt both cards and tap them gently. Watch the flour.

What you will see: The flour slides off the dry card. It sticks to the oiled card. Mucus is sticky in the same way, so it holds dust and pathogens until the cilia sweep them away. This is only a model. Real mucus is not oil.

If you cannot try it, this is what happens.
A table with the columns Card, Flour after tilting. dry, slides off. oiled, sticks.CardFlour after tiltingdryslides offoiledsticks

A wrong idea many people have

Antibodies are cells that swallow pathogens.

Why it feels right: The words antibody and white blood cell are used together, and both fight pathogens.

The test that breaks it: Look at the picture of the antibody and the picture of the phagocyte. An antibody is a small protein with a Y shape. A phagocyte is a whole cell with a nucleus.

What is true: An antibody is a protein made by a white blood cell. It sticks to an antigen. A phagocyte is a white blood cell that takes pathogens in.

Key words

mucus
a sticky liquid in the airways that traps pathogens and dust.
cilia
tiny hairs on cells that sweep mucus out of the airways.
white blood cells
cells in the blood that defend the body against pathogens.
phagocyte
a white blood cell that takes in a pathogen and digests it.
antigens
chemicals on the surface of a pathogen that the body does not make.
antibodies
proteins that fit one antigen and stick to the pathogen.
memory cells
white blood cells that stay in the blood and remember an antigen.
immunity
protection from a disease, because the body can make the right antibody quickly.

Back to your guess

The answer is A: The body has barriers that keep most pathogens out, and cells that destroy the ones that get in.

Pathogens do get on and into the body every day. The body has two lines of defence. The first keeps most of them out. The second is made of white blood cells, which destroy the ones that get in.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which defence kills pathogens that are swallowed?

    Tap your answer.

    1. Not this one. Skin keeps pathogens out of the body. It does not kill the ones that are swallowed. Try another.

    2. Not this one. Cilia sweep mucus out of the airways. They do not kill pathogens. Try another.

    3. Yes, that is right. The stomach makes a strong acid that kills most of the pathogens that reach it in food, drink and swallowed mucus.

    See the answer

    Stomach acid

    The stomach makes a strong acid that kills most of the pathogens that reach it in food, drink and swallowed mucus.

    • If you chose A: Skin keeps pathogens out of the body. It does not kill the ones that are swallowed.
    • If you chose B: Cilia sweep mucus out of the airways. They do not kill pathogens.

  2. What do cilia do in the airways?

    Tap your answer.

    1. Not this one. Antibodies are made by white blood cells. Cilia are tiny hairs on cells that line the airways. Try another.

    2. Not this one. Acid is made in the stomach. Cilia do not kill pathogens. Try another.

    3. Not this one. Oxygen enters the blood in the lungs, through very thin walls. Cilia do not do that. Go to S3.3.5. Try another.

    4. Yes, that is right. Mucus traps pathogens and dust. Cilia beat together and move the mucus up to the throat, where it is swallowed or coughed out.

    See the answer

    They sweep the mucus, with the pathogens in it, out of the airways.

    Mucus traps pathogens and dust. Cilia beat together and move the mucus up to the throat, where it is swallowed or coughed out.

    • If you chose A: Antibodies are made by white blood cells. Cilia are tiny hairs on cells that line the airways.
    • If you chose B: Acid is made in the stomach. Cilia do not kill pathogens.
    • If you chose C: Oxygen enters the blood in the lungs, through very thin walls. Cilia do not do that. Go to S3.3.5.

  3. What does a phagocyte do?

    Tap your answer.

    1. Not this one. Other white blood cells make antibodies. A phagocyte takes pathogens in. Try another.

    2. Yes, that is right. A phagocyte reaches out, wraps round a pathogen and takes it inside. Enzymes then break the pathogen down.

    3. Not this one. Red blood cells carry oxygen. A phagocyte is a white blood cell. Go to S3.3.6. Try another.

    4. Not this one. Mucus is made by cells that line the airways. A phagocyte is a white blood cell. Try another.

    See the answer

    It takes a pathogen into itself and digests it.

    A phagocyte reaches out, wraps round a pathogen and takes it inside. Enzymes then break the pathogen down.

    • If you chose A: Other white blood cells make antibodies. A phagocyte takes pathogens in.
    • If you chose C: Red blood cells carry oxygen. A phagocyte is a white blood cell. Go to S3.3.6.
    • If you chose D: Mucus is made by cells that line the airways. A phagocyte is a white blood cell.

  4. Look at the picture. Why will these antibodies not stick to the pathogen?

    A pathogen drawn as a dotted rounded block with four triangle shaped bumps on its top. Two Y shaped antibodies hover above it with a gap. The tip of each antibody arm is cut to a square shape, which is not the shape of a triangle. The labels are antibody, antigen and pathogen.antibodyantigenpathogen
    Tap your answer.

    1. Not this one. The size of the pathogen does not matter. Antibodies stick to antigens on its surface. Try another.

    2. Yes, that is right. An antibody fits only one kind of antigen. These tips are square, and the antigens are triangles, so they do not match.

    3. Not this one. Antibodies stick to living pathogens. They help the body to destroy them. Try another.

    See the answer

    The tips of the antibodies are not the shape of the antigens.

    An antibody fits only one kind of antigen. These tips are square, and the antigens are triangles, so they do not match.

    • If you chose A: The size of the pathogen does not matter. Antibodies stick to antigens on its surface.
    • If you chose C: Antibodies stick to living pathogens. They help the body to destroy them.

  5. Why is a person usually not ill the second time the same pathogen gets in?

    Tap your answer.

    1. Not this one. The antibody level falls after an infection. What stays are memory cells, which can make antibodies again quickly. Try another.

    2. Not this one. The skin does not change. The second response is made by white blood cells. Try another.

    3. Not this one. Memory cells remember only the antigen they met. They do not protect against other pathogens. Try another.

    4. Yes, that is right. Memory cells stay in the blood after an infection. If the pathogen returns, they make the right antibody sooner and in larger amounts.

    See the answer

    Memory cells make the right antibodies quickly and in large amounts.

    Memory cells stay in the blood after an infection. If the pathogen returns, they make the right antibody sooner and in larger amounts.

    • If you chose A: The antibody level falls after an infection. What stays are memory cells, which can make antibodies again quickly.
    • If you chose B: The skin does not change. The second response is made by white blood cells.
    • If you chose C: Memory cells remember only the antigen they met. They do not protect against other pathogens.

  6. Which statement about an antibody is true?

    Tap your answer.

    1. Yes, that is right. The tip of an antibody has a shape that fits one antigen only, as a key fits one lock. That is why the body needs a different antibody for each pathogen.

    2. Not this one. An antibody has a tip of one shape, so it fits only one kind of antigen. Try another.

    3. Not this one. A phagocyte is a cell that takes in pathogens. An antibody is a protein. Try another.

    4. Not this one. Antibodies are made by the body's white blood cells, not by the pathogen. Try another.

    See the answer

    It fits one kind of antigen.

    The tip of an antibody has a shape that fits one antigen only, as a key fits one lock. That is why the body needs a different antibody for each pathogen.

    • If you chose B: An antibody has a tip of one shape, so it fits only one kind of antigen.
    • If you chose C: A phagocyte is a cell that takes in pathogens. An antibody is a protein.
    • If you chose D: Antibodies are made by the body's white blood cells, not by the pathogen.

  7. From an earlier chapter

    Which control cuts the spread of cholera?

    Tap your answer.

    1. Not this one. A bed net stops mosquitoes. Cholera does not spread by mosquitoes. Go to S4.4.2. Try another.

    2. Yes, that is right. Cholera spreads through water that has faeces in it. Toilets and drains keep the faeces away from the water people drink.

    3. Not this one. That cuts the air route, for diseases such as influenza. Cholera spreads through water. Go to S4.4.2. Try another.

    See the answer

    Toilets that keep faeces away from drinking water

    Cholera spreads through water that has faeces in it. Toilets and drains keep the faeces away from the water people drink.

    • If you chose A: A bed net stops mosquitoes. Cholera does not spread by mosquitoes. Go to S4.4.2.
    • If you chose C: That cuts the air route, for diseases such as influenza. Cholera spreads through water. Go to S4.4.2.

  8. A count of 1 mm³ of blood finds 5 000 000 red blood cells and 8 000 white blood cells. About how many red cells are there for each white cell?

    See the answer

    about 625 red cells

    5 000 000 ÷ 8 000 = 625.

  9. After a first infection the antibody level peaks at 20. After a second infection by the same pathogen it peaks at 60. How many times higher is the second peak?

    See the answer

    3 times

    60 ÷ 20 = 3.

Keep it

Copy this into your exercise book.

First line: skin, mucus with cilia, stomach acid. Second line: white blood cells. Phagocytes take pathogens in. Antibodies fit one antigen. Memory cells give a faster, bigger response next time.

Draw: A pathogen with a triangle antigen, and a Y shaped antibody with a triangle tip fitted onto it.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.