The specific immune responseEdexcel A-Level Biology A: Subtopic test

1. 1
   Antibodies are Y-shaped proteins made by plasma cells. Each antibody molecule is made of four polypeptide chains and has two identical antigen-binding sites. An antibody made against an influenza virus protein was tested and found to bind only to that influenza protein.

   (a)

   Which part of an antibody molecule is responsible for its specificity for one antigen?

   [1 mark]

   • AThe constant region at the base of the stem
   • BThe variable region at the tips of the arms
   • CThe disulfide bridges between the chains
   • DThe hinge region in the middle of the molecule

   (b)

   Which type of bond holds the four polypeptide chains of an antibody together?

   [1 mark]

   • ADisulfide bridges
   • BPeptide bonds between the chains
   • CGlycosidic bonds
   • DPhosphodiester bonds

   (c)

   Explain why this antibody does not bind to a protein on the surface of a measles virus.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient is infected with a virus. Macrophages in the tissues engulf viral particles, and some days later killer T cells are found destroying virus-infected body cells.

   (a)

   How does a macrophage act as an antigen-presenting cell?

   [1 mark]

   • AIt secretes antibodies against the virus
   • BIt transfers viral RNA to a T cell
   • CIt destroys the virus completely so no antigens remain
   • DIt displays viral antigens on its cell surface membrane after digesting the virus

   (b)

   Which substance do killer T cells release to destroy an infected body cell?

   [1 mark]

   • AHistamine, which makes the cell leaky
   • BAntibodies, which bind to the cell
   • CPerforin, which makes holes in the cell surface membrane
   • DInterferon, which digests the cell

   (c)

   Describe the role of helper T cells in the immune response to this virus.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Patient P has a rare inherited condition in which no B cells develop, although T cells are normal. Patient Q has a different condition in which helper T cells are absent, although B cells are normal. Both patients suffer repeated infections.

   (a)

   Describe how a B cell is activated and produces antibodies against an antigen.

   [3 marks]

   (b)

   Explain why patient P cannot make antibodies and why patient Q also makes very few antibodies.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A child is infected with a virus that circulates in the blood and also infects body cells. The child's immune system makes antibodies against a protein on the surface of the virus, and also destroys infected body cells.

   (a)

   Compare the roles of B cells and T cells in the specific immune response to this virus.

   [6 marks]

   (b)

   Explain how a macrophage, a helper T cell and a B cell cooperate to produce antibodies against the viral surface protein.

   [6 marks]

   Total for question 4: 12 marks

End of questions