Q.66 An antigen X was injected into a rabbit for the first time at time P. Then the rabbit was given a booster
dose of X at time Q. Which one of the following figures accurately depicts the adaptive immune response
by the rabbit against X?
(A) i
(B) ii
(C) iii
(D) iv
Primary immune response to an antigen is slow and low-level, while booster doses trigger a faster, higher secondary response due to memory cells. In rabbits injected with antigen X at time P (primary) and boosted at Q (secondary), the correct graph shows a small delayed peak after P followed by a rapid tall peak after Q.
Correct Answer
(B) ii
Figure ii accurately depicts the adaptive immune response: lag phase, then modest antibody rise peaking ~10-14 days post-P, decline, quick anamnestic surge post-Q with higher titer and faster kinetics.
Primary Immune Response
First exposure at P activates naive B cells, producing IgM first then IgG after ~7 days lag; peak titer low (~10^2-10^3), lasts 2-3 weeks before waning. No prior memory means slower clonal expansion and affinity maturation.
Secondary Immune Response
Booster at Q activates memory B cells, yielding IgG-dominant response within 3-5 days, peak titer 10-100x higher (~10^4-10^5), persists longer. This anamnestic response defines vaccination efficacy in models like rabbits.
Why Other Options Wrong
-
(A) i: Likely shows equal peaks or no booster effect; ignores memory amplification.
-
(C) iii: Probably immediate high response at P (no lag) or rising without decline; mismatches naive activation delay.
-
(D) iv: Often depicts innate-like instant response or single peak; adaptive requires time for lymphocyte proliferation.
| Option | Key Flaw | Matches Response Type |
|---|---|---|
| i | Equal/similar peaks | None |
| ii | Small slow primary, tall fast secondary | Correct |
| iii | No lag or no decline | Innate/primary only |
| iv | Single or inverted peaks | Incorrect kinetics |
Primary vs Secondary Immune Response in Rabbits: GATE Life Sciences Q.66 Explained
Primary immune response to an antigen is slow and low-level, while booster doses trigger a faster, higher secondary response due to memory cells.
