B lymphocytes and the humoral responseAQA A-Level Biology: Subtopic test

1. 1
   A patient was infected with a bacterium for the first time. A laboratory measured the concentration of a specific antibody against the bacterium in samples of her blood. No antibody was detectable for the first few days; it then rose and peaked after about two weeks.

   (a)

   Which type of cell secretes the antibody into the patient's blood?

   [1 mark]

   • AA phagocyte
   • BA cytotoxic T cell
   • CA memory cell
   • DA plasma cell

   (b)

   Which part of the antibody molecule makes it specific for the bacterium's antigen?

   [1 mark]

   • AThe constant region, which has a tertiary structure complementary to the antigen
   • BThe disulfide bridges, which join the polypeptide chains together
   • CThe variable region, which has a tertiary structure complementary to the antigen
   • DThe hinge region, which has a tertiary structure complementary to the antigen

   (c)

   Explain why an antibody against this bacterium does not bind to the antigens of other pathogens.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A biochemist analysed an antibody molecule from a patient's blood. The molecule consisted of four polypeptide chains: two identical long chains and two identical short chains. The chains were held together by bonds between sulfur atoms of cysteine residues, and each molecule had two identical sites that bound to the antigen.

   (a)

   What type of bond holds the polypeptide chains of the antibody together?

   [1 mark]

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

   (b)

   Which feature of this antibody makes it able to cause agglutination of bacteria?

   [1 mark]

   • AIt has four polypeptide chains, so it can digest the bacteria
   • BIt has two antigen-binding sites, so one antibody can bind to two bacteria
   • CIt has identical short chains, so it makes holes in the bacterial membrane
   • DIt has disulfide bridges, so it binds to phagocytes only

   (c)

   Explain how agglutination of bacteria by antibodies helps to destroy them.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A child was exposed to the measles virus for the first time at the age of two. The concentration of anti-measles antibody in the blood rose slowly and peaked at about 14 days, and the child had a fever and a rash. At the age of six she was exposed to the virus again. Antibody appeared in her blood within two days, reached a concentration about 100 times higher, and she showed no symptoms.

   (a)

   Explain the differences between the antibody responses to the first and second exposures to the virus.

   [3 marks]

   (b)

   Explain why the child showed symptoms after the first exposure but not after the second.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient was infected with Streptococcus pneumoniae (strain X). She recovered after about two weeks. A year later she was exposed to strain X again and showed no symptoms. Some months after that she was exposed to a different strain, strain Y, which has a differently shaped surface protein, and became ill.

   (a)

   Explain how the humoral response led to the destruction of strain X bacteria during the first infection.

   [6 marks]

   (b)

   Explain why the patient did not become ill when exposed to strain X for a second time, but did when exposed to strain Y.

   [6 marks]

   Total for question 4: 12 marks

End of questions