Immunity and antibiotics

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   Explain how a population of bacteria can become resistant to an antibiotic.

   Mark scheme

   • A random mutation gives some bacteria an allele for resistance (1)
   • When the antibiotic is used, susceptible bacteria are killed but resistant bacteria survive (1)
   • Resistant bacteria reproduce and pass on the resistance allele (including by horizontal transfer on plasmids) (1)
   • So the frequency of the resistance allele increases in the population (1)
2. Question 25 marks

   Describe how B cells are activated and produce antibodies during a primary immune response.

   Mark scheme

   • A macrophage engulfs the pathogen and presents its antigens on its surface (antigen-presenting cell) (1)
   • A T helper cell with a complementary receptor binds to the presented antigen and is activated (1)
   • The T helper cell releases cytokines that activate a B cell with a complementary antibody receptor (that has bound the antigen) (1)
   • The B cell divides by mitosis (clonal expansion) and differentiates into plasma cells (1)
   • Plasma cells secrete antibodies specific to the antigen; some B cells become memory cells (1)