Antibody Response After Vaccination in an Older Adult
A 78-year-old man receives a two-dose vaccine schedule. Blood is sampled before dose 1, 10 days after dose 1, and 14 days after dose 2. Results: pre-dose 1, antibody titre below detection; day 10 after dose 1, IgM present but IgG low; 14 days after dose 2, IgG titre high and IgM low. A younger colleague vaccinated on the same days shows a higher IgG titre after dose 2. The older man has mild chronic kidney disease but no immunosuppressive medication. He asks whether the vaccine 'did not work' because he had no antibody on day 10.
1. Explain what the day-10 and post-dose-2 results show about the primary and secondary responses, and why the day-10 result does not mean failure.
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The day-10 sample shows an early primary response: IgM appears first, and IgG is only beginning to rise. This is expected - the primary response has a lag phase while antigen-presenting cells, helper T cells and B cells interact, and IgM-secreting plasma cells appear before class switching to IgG. Sampling at 10 days catches the response mid-development.
The post-dose-2 sample shows a secondary response: high IgG, low IgM. This pattern reflects memory B cells generated after dose 1 being rapidly reactivated by dose 2, producing a faster, larger, class-switched IgG response.
So the day-10 absence of IgG is not evidence of failure - it is evidence that the response was still developing. The correct judgement is to interpret each sample against the expected kinetics of that stage, not to compare a day-10 sample to a post-booster sample.
2. The older man's titre is lower than his younger colleague's. Explain possible reasons, and state what conclusion can and cannot be drawn about his protection.
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Possible reasons for a lower titre:
- Immunosenescence: ageing reduces the diversity and efficiency of T-cell and B-cell responses, so the magnitude of antibody production after vaccination is often lower.
- Chronic kidney disease: uraemia is associated with impaired immune function, which can blunt vaccine responses.
- Timing and individual variation: titre measured at a single time point varies between individuals.
What can be concluded: his immune system responded - he produced IgM then class-switched IgG, and the booster amplified IgG. The vaccine engaged his adaptive immune system.
What cannot be concluded: that he is unprotected. Antibody titre is one correlate of protection, not a direct measure of it. Cell-mediated immunity and memory B cells are not captured by a titre, and a lower titre may still be adequate. Equally, one cannot conclude he is fully protected. The honest judgement is that the result is consistent with a real but weaker response, and clinical advice should reflect uncertainty rather than declaring success or failure from a single number.