Primary and Booster Antibody Responses in a Child
A 10-year-old child is given a primary dose of an inactivated subunit vaccine on day 0. Blood samples show antibody titre: day 0 <1 unit, day 7 2 units, day 14 20 units, day 21 40 units, day 60 12 units, day 90 6 units. A booster is given on day 180. After the booster, the titre rises to 5 units by day 184 and 160 units by day 190, then falls slowly. The child's sibling, aged 2 years, received the same vaccine but no booster yet. The parents ask why the booster is needed, why it is not given on day 7 when the child was still making antibody, and whether the 2-year-old is protected. The clinic has no ability to measure memory B-cell numbers, only serum antibody.
1. Compare the primary and secondary responses using the titre data. Explain the differences in lag, peak and duration in terms of B cells and memory cells.
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Primary response: after day 0 there is a lag of about a week before antibody is detectable (2 units on day 7), a peak of 40 units around day 21, and a decline to 6 units by day 90. Secondary response: after the day-180 booster, antibody is detectable by day 184 (4 days), reaches 160 units by day 190, and falls more slowly. The primary lag reflects the time needed for antigen presentation, B-cell activation, clonal expansion and differentiation into antibody-secreting plasma cells. The secondary response is faster and larger because a pool of memory B cells specific for the vaccine antigen is already present; these respond to a lower antigen dose, proliferate rapidly and produce higher-affinity antibody, and some become long-lived plasma cells. The higher peak and slower decline indicate both more antibody and better maintenance.
2. Explain why giving the booster on day 7 would be unlikely to produce the day-190 pattern. What immunological principle does this illustrate?
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On day 7 the primary response is just beginning; memory B cells have not yet been generated in sufficient numbers and affinity maturation in germinal centres is incomplete. A booster at that time would be seen largely as more of the same primary antigen exposure, and the injected antigen might be cleared before a memory pool is established. The rapid, high secondary peak requires a pre-existing memory population. This illustrates the principle that boosters must be spaced after the primary response has generated memory, typically weeks to months later, not during the initial expansion phase. The exact interval is empirical, but the day-180 timing in the data allows the memory compartment to mature.
3. The 2-year-old sibling has only the primary dose. Predict the child's likely response to natural infection 1 year after vaccination compared with an unvaccinated child, and state what the antibody titre alone cannot tell you.
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If the primary dose generated memory, the vaccinated 2-year-old would likely mount a faster and stronger antibody response on natural infection than an unvaccinated child, with a shorter lag and a higher peak-a secondary-type response. However, protection depends on the pathogen, the correlate of protection, and the child's age and immune maturation. Serum antibody titre alone cannot measure the memory B-cell pool or T-cell memory, and a low titre does not necessarily mean no protection because memory cells can respond rapidly. Conversely, a high titre may wane. The clinic's inability to measure memory cells means it cannot directly confirm the cellular basis of protection; it can only infer from antibody kinetics and clinical data.