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B5.7 Human Defence Responses

GCSE Combined Science · AQA · Chapter B5: Communicable Diseases

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: non-specific first-line defences, and how white blood cells destroy pathogens by phagocytosis, antibodies and antitoxins. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how the skin, nose, trachea, bronchi and stomach stop pathogens entering the body
  • Explain the difference between physical and chemical barriers to infection
  • Describe phagocytosis by phagocytes
  • Explain how lymphocytes recognise antigens and produce specific antibodies and antitoxins
  • Explain why a second infection by the same pathogen is dealt with quickly

Key words

  • Non-specific defence: A defence that works against all pathogens, e.g. the skin or stomach acid.
  • Phagocyte: A white blood cell that engulfs and digests pathogens (phagocytosis).
  • Lymphocyte: A white blood cell that produces antibodies and antitoxins.
  • Antigen: A protein on the surface of a pathogen that the immune system recognises as foreign.
  • Antibody: A protein made by lymphocytes with a shape that fits one specific antigen.
  • Antitoxin: A protein made by lymphocytes that neutralises a toxin.

The first line of defence

These non-specific defences stop pathogens getting into the body in the first place. Some are physical barriers (skin, hairs, mucus, cilia); others are chemical barriers (stomach acid, antimicrobial secretions such as tears).

  • Skin; How it defends A tough physical barrier. Glands produce antimicrobial secretions. Cuts are sealed quickly by blood clots and scabs.
  • Nose; How it defends Hairs and mucus trap particles that may contain pathogens.
  • Trachea and bronchi; How it defends Goblet cells secrete mucus that traps pathogens. Cilia on ciliated cells beat to move the mucus up to the throat, where it is swallowed.
  • Stomach; How it defends Produces hydrochloric acid, which kills most pathogens in food and drink, and in swallowed mucus.

Key fact: Smoking stops cilia beating, so mucus and pathogens collect in the airways. This is one reason smokers get more chest infections.

The second line of defence: white blood cells

If a pathogen gets past the first line of defence and enters the blood or tissues, the immune system tries to destroy it. White blood cells do this in three ways.

1. Phagocytosis. Phagocytes are attracted to infected areas. They engulf pathogens into a vacuole and digest them with enzymes.

2. Producing antibodies. Lymphocytes recognise the foreign antigens on a pathogen and produce antibodies. Each antibody is specific: its shape fits only one antigen. Antibodies clump pathogens together, so phagocytes can engulf them more easily. Making enough antibodies takes a few days, which is why you feel ill for a while.

3. Producing antitoxins. Lymphocytes also make antitoxins, which bind to and neutralise the toxins released by some bacteria.

Remembering a pathogen

After an infection, memory lymphocytes stay in the blood. If the same pathogen enters again, they produce the right antibodies much faster and in larger numbers, so the pathogen is destroyed before you become ill. You are immune. Vaccination uses this (see B6.1).

You can still catch many colds, because there are hundreds of different cold viruses with different antigens.

Worked example: percentage increase

A patient's white blood cell count rises from 6.5 to 15.6 thousand per mm³ during an infection.
Increase = 15.6 − 6.5 = 9.1. Percentage increase = 9.1 ÷ 6.5 × 100 = 140%.

Answer: 140% increase

Common mistakes

  • Saying cilia trap pathogens. Mucus traps them; cilia move the mucus.
  • Saying white blood cells "eat" pathogens. Say phagocytes engulf and digest them.
  • Mixing up antigens (on the pathogen), antibodies (bind to antigens) and antitoxins (neutralise toxins).
  • Saying one antibody works against many pathogens. Each antibody is specific to one antigen.
  • Saying stomach acid digests food. It kills pathogens and gives the right pH for the stomach enzyme.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows the lining of the trachea. Which label points to a cell that makes mucus? (Choose from: A, B, C.)

Answer: C

Working: C is a goblet cell, which makes mucus. A is the layer of mucus with trapped particles. B points to the cilia, which beat to move the mucus up to the throat.

Question 2. The diagram shows three stages of phagocytosis in mixed order. What is the correct order? (Choose from: P, Q, R, R, P, Q, Q, R, P, R, Q, P.)

Answer: R, P, Q

Working: R: the white blood cell meets the pathogen. P: it engulfs (surrounds) it. Q: enzymes digest the pathogen inside a vacuole.

Question 3. True or false? An antibody made against one pathogen will also bind to a different pathogen.

Answer: False

Working: False. Antibodies are specific. Each one has a shape that fits only one antigen, so it binds only to that pathogen.

Related lessons

Continue in the full course

This lesson is section B5.7 (AGCSC0507) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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