The diagram shows the concentration of an antibody to a particular pathogen - HSC - SSCE Biology - Question 16 - 2019 - Paper 1

Since Student X was previously vaccinated, their immune system would have memory cells that respond quickly to the pathogen upon re-exposure. This results in a higher concentration of antibodies. In contrast, Student Y, having never been exposed, would have a much lower concentration of antibodies due to the lack of prior immune response.

Based on the graph, we can infer that:

• Student X will likely have a higher antibody concentration of around 1500 mg L⁻¹.
• Student Y will likely have an antibody concentration of around 250 mg L⁻¹.

Thus, the correct answer is option D:

Concentration of antibody in the blood (mg L⁻¹)
Student X: 1500
Student Y: 250