GCSE · Biology · AQA · Spec 8461

Human defence systems

Every breath, every mouthful and every scraped knee is a possible way in for a pathogen. So what's waiting on each route? Let's find out: you're the pathogen.

Biology · The first line of defence

Be the pathogen: pick a way in

You're a pathogen trying to get into a human body. Choose your way in, keep choosing, and see what's waiting for you.

6 ways your invasion can end.

On You're a pathogen looking for a way in. 3 branches to choose from.

Every barrier you met here works against any pathogen that tries its route. And anything that gets past them and inside the body meets the second line of defence: the immune system and its white blood cells.

Watch out: Cilia don't kill pathogens. They move the mucus that has trapped them. On the airway route, the killing happens later, in the stomach.

Biology · Phagocytosis

How a phagocyte eats a pathogen

The order a phagocyte does these things in

1 · First4 · Last

  1. Releases enzymes that digest and destroy the pathogen

  2. Takes the pathogen inside itself

  3. Surrounds the pathogen

  4. Changes shape

Watch out: The enzymes come last. The phagocyte digests the pathogen after it has taken it inside, not while it is still outside the cell.

The antibody route, step by step

Watch who does the destroying at the end. It isn't the antibodies.

  1. Antigens on the surfaceEvery pathogen has antigen molecules on its outer surface. Think of them as the pathogen's badge.
  2. Lymphocytes reactLymphocytes, another kind of white blood cell, react to those antigens by making antibodies.
  4. Pathogens clumpAntibodies that attach to pathogens make them stick together in clumps.
  5. Phagocytes finish the jobPhagocytes engulf and destroy the clumps. The antibodies never destroyed anything themselves. They caught the pathogens and bundled them up for the phagocytes.

Antibodies vs antitoxins

AntibodiesvsAntitoxins

Start with the first row. Get the target right and the rest follows.

Focus

What each one is matched to

Antibodies

One particular antigen, on the surface of a pathogen

Antitoxins

One particular toxin, a poison some bacteria release that damages body tissues

The insight

This is the difference to lead with. Antibodies go after the pathogen itself. Antitoxins go after the poison it has released.

What it does

Antibodies

Binds to the pathogens that carry its antigen and clumps them together

Antitoxins

Neutralises its toxin so the toxin no longer causes harm

Made by

Antibodies

Lymphocytes, a kind of white blood cell

Antitoxins

White blood cells

Biology · Two kinds of defence

Every pathogen, or just one?

Does each defence work against every kind of pathogen, or is it matched to one particular antigen or toxin?

Still to sort

Works against every kind of pathogen (0)

These are the non-specific defences

Where the line is: The line is not 'barriers versus white blood cells'. Phagocytosis is done by white blood cells, and it's still non-specific.

Matched to one antigen or toxin (0)

Each one fits a single target

8 of 8 still to sort.

You've now met every defence in this topic. Sort them, then read why each belongs where it does.

Biology · Put it into words

Write it, then mark it

Explain the role of the immune system in the defence against disease. [6 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Barriers keep pathogens out. White blood cells destroy the ones that get in.

What you need to know

  • Non-specific defences work against every kind of pathogen. They include skin, the nose, the trachea and bronchi, and the stomach.
  • Skin is a physical barrier. If it's cut, platelets clump and form a clot that seals the wound (a scab).
  • In the nose, tiny hairs and mucus trap dust and dirt that may carry pathogens.
  • In the trachea and bronchi, goblet cells make mucus that traps pathogens, and cilia sweep it up to the throat to be swallowed.
  • Hydrochloric acid in the stomach kills pathogens swallowed with food or in mucus from the airways.
  • If a pathogen gets inside the body, the immune system tries to destroy it. White blood cells do this by phagocytosis, antibody production and antitoxin production.

The big picture

Your body defends itself in two lines. First, a barrier waits on every way in: skin (and a clot if it's cut), hairs and mucus in the nose, mucus and cilia in the trachea and bronchi, and acid in the stomach. These are non-specific: they work against every kind of pathogen. Second, if a pathogen does get inside, the immune system tries to destroy it, and white blood cells do the work in three ways: phagocytosis, making antibodies and making antitoxins.

Key points

1The first line of defence is a set of barriers, one on each route in. They are non-specific.
2Mucus from the airways is swallowed, so the airway route and the food route both end in stomach acid.
3Phagocytosis: a phagocyte changes shape, surrounds a pathogen, takes it inside and releases enzymes to digest it. It is non-specific.
4Lymphocytes make antibodies. Each antibody's shape fits one antigen, so it binds only that pathogen and clumps pathogens for phagocytes to engulf.
5Antitoxins are each matched to one toxin, and they neutralise it so it no longer causes harm.

Worked example

Problem

A bacterium gets into the body through a cut. It has antigens on its surface, and it releases a toxin that damages body tissues. Which white blood cell defences act against the bacterium, and which act against its toxin?

⚠ Watch out

Saying that antibodies kill pathogens. They don't destroy anything themselves: each antibody binds to the antigen it fits and clumps the pathogens together, and then phagocytes engulf and destroy the clumps.

🧠

Memory hook

Keep out, then take out. Barriers keep pathogens out. White blood cells take out the ones that get in, in three ways: EAT them (phagocytosis), STICK them together (antibodies), and NEUTRALISE the poisons some bacteria release (antitoxins).

✓

Check yourself

Close the page. Name the defence on each route in and say what it does. Then say which white blood cell method is non-specific, and why the other two aren't.

Flashcards

(14)
What does 'non-specific defence' mean?
A defence that works against every kind of pathogen, not one particular type.
How does skin protect the body from pathogens?
It covers almost all of the body and acts as a physical barrier. A cut gives pathogens a way in.
What happens in the blood when skin is cut?
Platelets clump and form a clot that seals the wound. You see the clot as a scab.
How does the nose keep pathogens out of the gas exchange system?
Tiny hairs filter the air and, with mucus, trap dust and dirt that may carry pathogens. The mucus can be blown out.
What do goblet cells in the trachea and bronchi do?
They make mucus that traps pathogens from breathed-in air.
What do the cilia in the trachea and bronchi do?
They sweep mucus up to the throat so it can be swallowed.
How does the stomach defend against pathogens?
It contains hydrochloric acid, which kills pathogens swallowed with food or in mucus from the airways.
What happens if a pathogen gets inside the body?
The immune system tries to destroy it, and white blood cells carry out that defence.
What are the steps of phagocytosis?
A phagocyte changes shape, surrounds the pathogen, takes it inside, then releases enzymes that digest and destroy it.
Is phagocytosis specific or non-specific?
Non-specific: a phagocyte engulfs pathogens without matching one particular antigen.
What are antigens, and what do lymphocytes do when they meet them?
Antigens are molecules on a pathogen's outer surface. Lymphocytes react to them by making antibodies.
Why does each kind of antibody bind to only one kind of pathogen?
An antibody is a protein whose shape fits one particular antigen, so it binds only to pathogens carrying that antigen.
What happens to pathogens once antibodies attach to them?
They stick together in clumps, which phagocytes can then engulf and destroy.
What is a toxin, and what does an antitoxin do to it?
A toxin is a poison some bacteria release that damages body tissues. An antitoxin, matched to that one toxin, neutralises it so it no longer causes harm.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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