B lymphocytes and the humoral responseAQA A-Level Biology: Flashcards

Exam questions on B lymphocytes and the humoral response

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.
   Explain why an antibody against this bacterium does not bind to the antigens of other pathogens.2 marks
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.
   Explain how agglutination of bacteria by antibodies helps to destroy them.2 marks
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.
   Explain the differences between the antibody responses to the first and second exposures to the virus.3 marks