Cell recognition and the immune systemAQA A-Level Biology: Topic test

1. 1
   The immune system distinguishes the body's own healthy cells from other cells by the molecules on their surface. A patient has four types of cell: his own healthy skin cells, melanoma cells with an abnormal protein on their surface, cells of a bacterium that has infected a wound, and cells in a skin graft taken from an unrelated donor.

   (a)

   Which of these cells would normally NOT stimulate an immune response in the patient?

   [1 mark]

   • AThe melanoma cells.
   • BThe bacterial cells.
   • CHis own healthy skin cells.
   • DThe cells of the skin graft.

   (b)

   Which is the best definition of an antigen?

   [1 mark]

   • AA foreign molecule, usually a protein, that stimulates an immune response.
   • BA protein made by B cells that binds to a specific pathogen.
   • CA white blood cell that engulfs and digests pathogens.
   • DAn enzyme in phagocytes that hydrolyses bacterial cell walls.

   (c)

   Explain why the skin graft from the unrelated donor would be rejected.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A phagocyte in a wound engulfs a bacterium. The bacterium is enclosed in a vesicle inside the phagocyte, and this vesicle then fuses with lysosomes.

   (a)

   Which sequence correctly describes phagocytosis of the bacterium?

   [1 mark]

   • AThe phagocyte attaches to the bacterium, lysosomes fuse with the cell-surface membrane, and the bacterium is digested outside the cell.
   • BThe bacterium is digested, then engulfed in a vesicle, and then lysosomes fuse with the vesicle.
   • CThe bacterium is engulfed in a vesicle, and lysosomes release lysozymes into the cytoplasm away from the vesicle.
   • DThe bacterium is engulfed in a vesicle, lysosomes fuse with the vesicle, and lysozymes hydrolyse the bacterium.

   (b)

   What is the role of the lysozymes in this process?

   [1 mark]

   • AThey engulf the bacterium.
   • BThey hydrolyse the bacterium, for example its cell wall.
   • CThey display antigens on the cell-surface membrane of the phagocyte.
   • DThey produce antibodies against the bacterium.

   (c)

   Explain how the phagocyte can then help to start a specific immune response.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A person is infected with a bacterium that has antigen Z on its surface. The person's blood contains millions of B cells, each carrying a different type of antibody on its surface membrane. Only about one B cell in a million has an antibody that binds to antigen Z.

   (a)

   Describe how antibodies against antigen Z come to be produced in large quantities.

   [3 marks]

   (b)

   Describe how the antibodies lead to the destruction of the bacteria.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A seven-year-old child has chickenpox, caused by a virus that infects skin cells. She recovers after ten days. A chickenpox vaccine contains a weakened form of the virus and is not given to her until after she has recovered.

   (a)

   Explain how the child's immune system destroyed the virus and the infected skin cells.

   [6 marks]

   (b)

   Explain how the vaccine would have protected her from chickenpox if she had been given it, and explain why antibiotics could not be used to treat chickenpox.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A newborn baby receives antibodies from her mother in breast milk. At eight weeks old she receives her first dose of a vaccine against a bacterial disease.

   (a)

   Which row correctly describes the baby's immunity from the breast milk and from the vaccine?

   [1 mark]

   • ABreast milk: passive immunity; vaccine: active immunity.
   • BBreast milk: active immunity; vaccine: passive immunity.
   • CBreast milk: active immunity; vaccine: active immunity.
   • DBreast milk: passive immunity; vaccine: passive immunity.

   (b)

   Why does the protection from the antibodies in breast milk last only a short time?

   [1 mark]

   • APlasma cells in the baby stop making the antibodies.
   • BThe antibodies are used up by T cells.
   • CThe antibodies in milk cannot bind to antigens.
   • DThe baby's B cells did not make them, so no memory cells form and the antibodies are gradually broken down.

   (c)

   Some people oppose compulsory vaccination. State two ethical issues associated with the use of vaccines.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A veterinary clinic tests dogs for a parasite using an ELISA. Each well of a plastic plate is coated with antigen from the parasite. Serum from a dog is added to a well, which is then washed. A second antibody, linked to an enzyme, that binds to dog antibodies is then added and the well is washed again. Finally a colourless substrate is added, which the enzyme converts into a coloured product.

   (a)

   What is the purpose of washing the well after the serum is added?

   [1 mark]

   • ATo kill any parasites in the serum.
   • BTo remove unbound antibodies and other serum components, so that only antibodies attached to the antigen remain.
   • CTo activate the enzyme on the second antibody.
   • DTo denature the antigen so that it sticks to the plastic.

   (b)

   A dog that has never been infected is tested. What result is expected, and why?

   [1 mark]

   • AColour develops, because the second antibody binds directly to the antigen on the plate.
   • BColour develops, because the substrate is converted without an enzyme.
   • CNo colour develops, because there are no antibodies to the parasite for the second antibody to bind to, so it is washed away.
   • DNo colour develops, because the enzyme cannot work in the presence of serum.

   (c)

   Explain why using an enzyme makes the test sensitive and allows the amount of antibody present to be estimated.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   HIV is a retrovirus. Each virus particle has an outer envelope carrying attachment proteins, a protein capsid, two strands of RNA and the enzyme reverse transcriptase.

   (a)

   Describe how HIV replicates once it has entered a helper T cell.

   [3 marks]

   (b)

   A person infected with HIV stays healthy for several years, but then suffers repeated infections and some cancers. Explain how HIV infection leads to these symptoms of AIDS.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   In a trial of a new vaccine against a bacterial disease, 20 000 volunteers were randomly divided into two equal groups. One group received the vaccine and the other group received an injection of saline (a placebo). Over the following year, 12 of the vaccinated volunteers and 96 of the volunteers who received the placebo developed the disease. Mild side effects, such as a sore arm or a slight fever, were reported by 18% of the vaccinated volunteers and by 4% of those given the placebo.

   (a)

   Evaluate the evidence that the vaccine is effective and safe.

   [6 marks]

   (b)

   The bacterium has several strains with different surface antigens, and the vaccine contains the antigens of only one strain. Explain why the vaccine may not protect against all strains.

   [6 marks]

   Total for question 8: 12 marks

End of questions