Final answer:
The second blue peak around day 40 represents the peak of the secondary immune response to antigen X, characterized by a rapid and robust production of IgG antibodies due to the activation of memory B cells.
Step-by-step explanation:
The second blue peak around day 40 likely represents the peak of the secondary immune response to antigen X. According to the information provided, when there's a subsequent exposure to the same antigen (antigen A in Figure 21.24, analogous to antigen X), the secondary immune response is triggered, leading to a quicker and more robust production of antibodies, particularly IgG, than during the primary response. IgG levels peak significantly higher in the secondary response compared to the primary response. This heightened response is due to the presence of memory B cells, which were generated during the primary immune response and facilitate a faster and more efficient response upon re-exposure to the same antigen.