A molecule, usually a protein, that stimulates an immune response.

What does the immune system identify using cell surface molecules?

Pathogens, cells from other organisms of the same species, abnormal body cells and toxins.

Why are transplanted organs rejected?

The donor cells have different surface proteins (antigens), recognised as non-self.

What is antigen variability?

Mutations in genes for a pathogen's antigens produce new antigens of a different shape.

How does antigen variability affect disease?

Memory cells from an earlier infection do not recognise the new antigens, so reinfection occurs.

How does antigen variability affect disease prevention?

Vaccines must be updated, and it is hard to make vaccines against variable pathogens.

A white blood cell, such as a neutrophil or macrophage, that engulfs and destroys pathogens.

The vacuole formed when a phagocyte engulfs a pathogen.

What do lysosomes release after fusing with a phagosome?

Lysozymes, which hydrolyse the pathogen.

What is the difference between lysosome and lysozyme?

A lysosome is an organelle containing enzymes; lysozyme is the hydrolytic enzyme inside.

Why is phagocytosis described as non-specific?

It works on many different pathogens, not just one type of antigen.

What happens to the products of digestion in a phagocyte?

Useful soluble products are absorbed into the cytoplasm.

Antigens and phagocytosisAQA A-Level Biology: Flashcards

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Define antigen.

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Define antigen.: A molecule, usually a protein, that stimulates an immune response.
What does the immune system identify using cell surface molecules?: Pathogens, cells from other organisms of the same species, abnormal body cells and toxins.
Why are transplanted organs rejected?: The donor cells have different surface proteins (antigens), recognised as non-self.
What is antigen variability?: Mutations in genes for a pathogen's antigens produce new antigens of a different shape.
How does antigen variability affect disease?: Memory cells from an earlier infection do not recognise the new antigens, so reinfection occurs.
How does antigen variability affect disease prevention?: Vaccines must be updated, and it is hard to make vaccines against variable pathogens.
What is a phagocyte?: A white blood cell, such as a neutrophil or macrophage, that engulfs and destroys pathogens.
What is a phagosome?: The vacuole formed when a phagocyte engulfs a pathogen.
What do lysosomes release after fusing with a phagosome?: Lysozymes, which hydrolyse the pathogen.
What is the difference between lysosome and lysozyme?: A lysosome is an organelle containing enzymes; lysozyme is the hydrolytic enzyme inside.
Why is phagocytosis described as non-specific?: It works on many different pathogens, not just one type of antigen.
What happens to the products of digestion in a phagocyte?: Useful soluble products are absorbed into the cytoplasm.
Name an example of a pathogen with variable antigens.: Influenza virus (or HIV).

Exam questions on Antigens and phagocytosis

Influenza viruses carry two types of glycoprotein on their surface, haemagglutinin and neuraminidase. The genes coding for these proteins mutate frequently. Every year public health scientists predict which strains will be common and a new seasonal influenza vaccine is produced.
Explain why a new influenza vaccine has to be produced each year.2 marks
A patient with kidney failure receives a kidney from an unrelated donor. Without drugs that suppress the immune system, the patient's immune system would attack the transplanted kidney.
Explain why finding a donor with a very close tissue match reduces the risk of the kidney being rejected.2 marks
Neutrophils, a type of phagocyte, were mixed with Staphylococcus aureus bacteria and viewed under a microscope. After a few minutes bacteria could be seen inside membrane-bound vacuoles in the neutrophils. A rare inherited condition causes a patient's neutrophils to engulf bacteria normally, but the bacteria survive inside the neutrophils and the patient suffers repeated infections.
Describe how a neutrophil ingests and destroys a bacterium.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).