5.9 Human Defence Responses

A free GCSE Biology lesson for AQA (8461) from OBA School: non-specific defences, the immune system, and how plant diseases are detected, identified and defended against. 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 how white blood cells defend the body by phagocytosis, antibodies and antitoxins
• Describe how plant diseases and ion deficiencies are detected and identified
• Describe the physical, chemical and mechanical defences of plants

Key words

• Non-specific defence: A defence that works against all pathogens, e.g. the skin.
• Phagocytosis: When a white blood cell engulfs a pathogen and digests it.
• Antigen: A molecule (usually a protein) on the surface of a pathogen that the immune system recognises.
• Antibody: A protein made by white blood cells that binds to one specific antigen.
• Antitoxin: A protein made by white blood cells that neutralises a toxin.
• Chlorosis: Yellowing of leaves caused by a lack of chlorophyll, e.g. from magnesium deficiency.

The first line of defence

These non-specific defences stop pathogens getting into the body in the first place.

• Skin; How it defends A tough physical barrier. It produces antimicrobial secretions, and healthy skin is covered in harmless microorganisms that compete with pathogens. Cuts are sealed 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 (tiny hairs) on the 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 damages the cilia, so mucus and pathogens collect in the airways. This is one reason smokers get more chest infections.

The immune system: white blood cells

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

1. Phagocytosis. Some white blood cells (phagocytes) engulf pathogens and digest them.

2. Producing antibodies. Other white blood cells (lymphocytes) recognise the antigens on a pathogen and make antibodies. Each antibody is specific: its shape fits only one antigen. Antibodies stick pathogens together or mark them, so phagocytes can destroy them more easily.

3. Producing antitoxins. These counteract (neutralise) the toxins that some bacteria release.

Key fact: If the same pathogen enters again, memory cells make the right antibodies very quickly, so you do not become ill. You are immune. Vaccination uses this (chapter 6).

Detecting and identifying plant diseases

Plants can be infected by viruses, bacteria and fungi, and attacked by insects such as aphids. Signs that a plant is diseased:

• stunted growth
• spots on leaves
• areas of decay (rot)
• growths
• malformed stems or leaves
• discolouration
• the presence of pests

To identify the disease, a gardener can use a gardening manual or website, take the infected plant to a laboratory, or use a testing kit containing monoclonal antibodies.

Some problems are caused by a lack of mineral ions, not by pathogens (these are not communicable):

• Nitrate; Why the plant needs it To make amino acids, which are used to make proteins for growth; Symptom Stunted growth
• Magnesium; Why the plant needs it To make chlorophyll; Symptom Chlorosis (yellow leaves)

How plants defend themselves

• Physical; Examples Cellulose cell walls; a tough waxy cuticle on leaves; layers of dead cells on the surface (such as bark) that fall off, taking pathogens with them.
• Chemical; Examples Antibacterial chemicals (e.g. in mint and pine resin); poisons that deter herbivores (e.g. in foxgloves and tobacco).
• Mechanical; Examples Thorns and hairs that deter animals; leaves that droop or curl when touched (e.g. the sensitive plant, Mimosa); mimicry to trick animals (e.g. leaves with dots that look like butterfly eggs).

Exam tip: Physical defences stop microorganisms getting in. Chemical defences kill pathogens or put off herbivores. Mechanical defences physically deter animals.

Common mistakes

• Saying cilia trap pathogens. Mucus traps them; cilia move the mucus.
• Saying white blood cells "eat" pathogens. Say they engulf and digest them (phagocytosis).
• Mixing up antibodies (bind to antigens on pathogens) and antitoxins (neutralise toxins).
• Saying one antibody works against many pathogens. Each antibody is specific to one antigen.
• Calling nitrate or magnesium deficiency a disease caused by a pathogen. It is caused by a lack of ions in the soil.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the job of the cilia in the trachea and bronchi? (Choose from: To trap pathogens, To move mucus up to the throat, To make mucus, To kill bacteria.)

Answer: To move mucus up to the throat

Working: Mucus traps pathogens; cilia beat to move the mucus up to the throat, where it is swallowed.

Question 2. What happens during phagocytosis? (Choose from: White blood cells make antibodies, White blood cells engulf and digest pathogens, Toxins are neutralised, Red blood cells destroy viruses.)

Answer: White blood cells engulf and digest pathogens

Working: In phagocytosis, a white blood cell surrounds (engulfs) a pathogen and digests it with enzymes.

Question 3. True or false? An antibody made against one pathogen will bind to every other pathogen.

Answer: False

Working: False. Each antibody is specific to one antigen, so it only binds to the pathogen with that antigen.

Related lessons

• Previous lesson: 5.8 Diseases Caused by Fungi and Protists
• Next lesson: 5.12 Required Practicals: Communicable Diseases
• In this chapter: 5.1 Health and Disease
• In this chapter: 5.2 Pathogens and Disease
• In this chapter: 5.3 Growing Bacteria in the Lab

Continue in the full course

This lesson is section 5.9 (AGCBH0509) of our free GCSE Biology 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 Biology course.