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AQA BiologyInfection and response

Human defence systems

Explain barriers, immune responses and antibody specificity.

Start here

The key idea

The body prevents pathogen entry with barriers and destroys pathogens using white blood cells.Antibodies bind to specific antigens.

Body defencesBarriers prevent entry; white blood cells respond inside the body.
Body defencesBarriers prevent entry; white blood cells respond inside the body.skinphysical barrierphagocyte engulfsantibodies
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

Non-specific defences

The body has several non-specific defences that stop pathogens entering.The skin acts as a barrier and produces antimicrobial secretions, while the nose has hairs and mucus to trap particles.The trachea and bronchi are lined with mucus and cilia that waste the mucus and trapped pathogens back up to be swallowed, and the stomach produces hydrochloric acid that kills most pathogens in food.

2

Phagocytosis

If a pathogen enters the body, white blood cells called phagocytes defend it non-specifically.A phagocyte engulfs the pathogen by surrounding it and then digests it using enzymes.This destroys the pathogen regardless of its type, providing a rapid first line of internal defence.

3

Antibody production

Other white blood cells called lymphocytes carry out a specific response.Every pathogen has unique molecules called antigens on its surface, and lymphocytes produce antibodies with a complementary shape that lock onto these antigens.The antibodies cause the pathogens to clump together so they are destroyed more easily, and this response is specific to one type of pathogen.

4

Antitoxins and immunity

Some lymphocytes produce antitoxins, which are proteins that bind to and neutralise the toxins released by bacteria.After an infection, some lymphocytes remain as memory cells.If the same pathogen enters again, these cells produce the correct antibody much more quickly and in larger amounts, so the person is immune and usually does not become ill.

Key terms

Definitions to learn

Phagocyte

A white blood cell that engulfs and digests pathogens.

Lymphocyte

A white blood cell that produces antibodies and antitoxins.

Antigen

A unique molecule on the surface of a pathogen that triggers an immune response.

Antibody

A protein produced by lymphocytes with a shape complementary to a specific antigen.

Antitoxin

A protein that binds to and neutralises a toxin produced by bacteria.

Memory cell

A lymphocyte that remains after infection and responds quickly if the same pathogen returns.

Worked example

Explain why a specific antibody may not bind to a different pathogen.

1

Each pathogen has antigens with a particular shape.

2

An antibody has a complementary binding site.

3

A different antigen shape will not fit.

Final answer

Antibodies are specific because their binding sites are complementary to particular antigens.

Exam habit

Describe the full sequence: pathogen enters → antigen recognised → antibodies produced → pathogen destroyed → memory cells remain.Antibodies are specific to one antigen — explain complementary shape.

Watch out

Do not say antibodies kill every pathogen directly. Explain binding and the immune response.

Examiner tips

How to score full marks

  • 1Antibodies are SPECIFIC — each one only fits one antigen; always mention the complementary shape.
  • 2Do not confuse antibodies (made by lymphocytes, lock onto antigens) with antitoxins (neutralise toxins) — examiners test the difference.
  • 3For immunity questions, the key idea is memory cells producing antibodies FASTER and in GREATER amounts on second exposure.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Give two physical or chemical barriers against infection.
Mark scheme
  1. 1.Think skin, mucus and stomach conditions.
Any two from: skin, mucus and cilia, stomach acid.
2State two ways white blood cells defend the body.
Mark scheme
  1. 1.Recall engulfing and antibody production.
Phagocytosis and producing antibodies or antitoxins.
3Why is a secondary immune response usually faster?
Mark scheme
  1. 1.Think memory cells.
Memory cells remain after the first exposure and rapidly produce the correct antibodies.
4Name the type of white blood cell that engulfs and digests pathogens.[1 mark]
Mark scheme
  1. 1.Recall the cell responsible for phagocytosis.
Phagocyte (1).
5State two non-specific defences that stop pathogens entering the body.[2 marks]
Mark scheme
  1. 1.Think about physical and chemical barriers.
Any two from: skin acting as a barrier (1); mucus and cilia in the airways (1); hydrochloric acid in the stomach (1); nose hairs / mucus trapping particles (1).
6Describe how a phagocyte destroys a pathogen.[2 marks]
Mark scheme
  1. 1.Describe the two stages of phagocytosis.
The phagocyte engulfs / surrounds the pathogen (1) and digests it using enzymes (1).
7Explain how antibodies help to destroy a specific pathogen.[4 marks]
Mark scheme
  1. 1.Link antigens, complementary shape and clumping.
Each pathogen has unique antigens on its surface (1); lymphocytes produce antibodies with a complementary shape (1) that lock onto the antigens, causing the pathogens to clump together (1) so they are destroyed / engulfed more easily (1).
8A person catches chickenpox once and is then immune for life. Explain, in terms of the immune system, why they do not become ill if exposed to the chickenpox virus again.[5 marks]
Mark scheme
  1. 1.Use memory cells and the speed and size of the second response.
During the first infection, lymphocytes produce antibodies and some remain as memory cells (1). When the same pathogen enters again, the memory cells recognise the antigens (1) and produce the correct antibodies much more quickly (1) and in much larger amounts (1), so the pathogen is destroyed before it can reproduce enough to cause symptoms (1).
9Explain the role of mucus and cilia in the airways in defending against pathogens.[4 marks]
Mark scheme
  1. 1.Describe how mucus traps pathogens.
  2. 2.Explain how cilia move the mucus.
Mucus traps pathogens and dust particles breathed in (1); cilia are hair-like structures that beat rhythmically (1) to move the mucus up towards the throat where it is swallowed (1); the pathogens are then destroyed by the acid in the stomach (1).
10Explain the difference between the roles of phagocytes and lymphocytes in the immune response.[4 marks]
Mark scheme
  1. 1.State the function of each cell type.
  2. 2.Note which is specific and which is non-specific.
Phagocytes carry out a non-specific response by engulfing and digesting any pathogen regardless of type (1); lymphocytes carry out a specific response by producing antibodies with a shape complementary to the specific antigen of that pathogen (1); phagocytes respond rapidly as a first line of defence (1); lymphocytes also produce memory cells for long-term immunity (1).
11A bacterium produces a toxin that makes a person feel ill. Explain how the immune system can neutralise the toxin.[3 marks]
Mark scheme
  1. 1.Recall the specific protein produced by lymphocytes in response to toxins.
Lymphocytes produce antitoxins (1); the antitoxin has a complementary shape to the toxin and binds to it (1); this neutralises the toxin so it can no longer damage cells (1).
12Monoclonal antibodies can be produced in a laboratory and used to target cancer cells. Suggest how an antibody could be used to deliver a drug directly to a tumour.[5 marks]
Mark scheme
  1. 1.Think about what makes antibodies specific to certain cells.
  2. 2.Explain how this specificity can be exploited.
Cancer cells display specific antigens on their surface (1); a monoclonal antibody is produced that is complementary to this antigen (1); the drug is attached to the antibody (1); when injected, the antibody binds only to the cancer cell antigen (1), delivering the drug directly to the tumour cells and minimising damage to healthy cells (1).
13Explain why a person who has recently had an organ transplant is given drugs to suppress their immune system, and state one risk of this treatment.[4 marks]
Mark scheme
  1. 1.Link the immune response to transplanted antigens.
  2. 2.State the consequence of a weakened immune system.
The transplanted organ carries antigens that are not the same as the patient's own antigens (1); the immune system would recognise these as foreign and mount an immune response to reject the organ (1); immunosuppressant drugs reduce this response, allowing the organ to be accepted (1); the risk is that the weakened immune system leaves the patient more vulnerable to infections (1).
14Describe the sequence of events from a pathogen entering the body for the first time to the person becoming immune to that pathogen.[6 marks]
Mark scheme
  1. 1.Start with the pathogen entering.
  2. 2.Include antigens, lymphocytes, antibody production and memory cells in sequence.
The pathogen enters the body and displays antigens on its surface (1); phagocytes engulf some pathogens as a rapid non-specific response (1); lymphocytes recognise the antigens and begin to divide (1); they produce specific antibodies complementary to the antigens (1); the antibodies cause pathogens to clump together and are destroyed (1); some lymphocytes remain as memory cells (1); on future exposure memory cells produce antibodies rapidly in large amounts, preventing illness (1). (Max 6.)
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