59. Three distinct antigens X, Y and Z
were used to raise antibodies. Antigen Z was injected in a mouse on
day zero followed by the administration of antigens X and
Y on day 28. A second injection of antigen X was
administered on day 70. The antibody titers were monitored in the serum every day
and the results are shown below.
Which one of the following statements regarding the antibody titers in the serum is
INCORRECT?
Correct Answer: (D) Y-specific IgG will be high on day 84 — INCORRECT
🔬 Introduction (SEO Optimized)
Understanding primary and secondary immune responses is a core concept in immunology and frequently tested in competitive exams like CSIR-NET, GATE, JAM, and NEET PG. This question examines how antibody titers (IgG) change over time when different antigens are introduced at different intervals. By analyzing the immune system’s memory response, we can identify which statement about antibody levels is incorrect.
🧬 Question Overview
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Antigen Z injected on Day 0
-
Antigens X and Y injected on Day 28
-
Antigen X injected again on Day 70
-
Antibody titers monitored daily
This setup helps distinguish between:
-
Primary immune response (first exposure)
-
Secondary immune response (re-exposure)
Immune Response Timeline (Conceptual)
| Antigen | Injection Day | Type of Response | Peak IgG |
|---|---|---|---|
| Z | Day 0 | Primary | ~Day 14 |
| X | Day 28 | Primary | ~Day 42 |
| Y | Day 28 | Primary | ~Day 42 |
| X (booster) | Day 70 | Secondary | ~Day 84 |
Option-wise Explanation
(A) Z-specific IgG will be high on day 14 ✅ Correct
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Antigen Z is injected on Day 0
-
Primary immune response peaks around Day 10–14
-
IgG production begins after IgM
✔️ Statement is correct
(B) X-specific antibody titer will be high on day 84 ✅ Correct
-
Antigen X is injected on Day 28 (primary response)
-
Booster dose on Day 70 triggers a secondary immune response
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Secondary responses produce higher and faster IgG peaks
✔️ Statement is correct
(C) X-specific IgG will be high on day 42 ✅ Correct
-
Day 42 is ~14 days after first exposure to X
-
This matches the peak of primary IgG response
✔️ Statement is correct
(D) Y-specific IgG will be high on day 84 ❌ Incorrect
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Antigen Y is injected only once on Day 28
-
No booster → no secondary immune response
-
By Day 84, IgG levels would have declined
❌ This statement is incorrect
Final Conclusion
The immune system produces high IgG titers only after recent exposure or re-exposure. Since antigen Y was not reintroduced, its IgG levels would not be high on Day 84.
Correct answer: (D)
