When looking at antibody count in the blood after vaccinations, why doesn't the count drop to zero?
Also for question 8 c. exam 1 2010, why is the antibody count in the baby above zero before the initial vaccine? (I deciphered this is because antibodies, through natural passive immunity, has reached the baby from the mother. But if the mother was only infected with measles as a child, why did she still have antibodies specific to measles after so many years?)
In Australia, vaccination against measles is a two-dose schedule. The first dose is generally given at age 12 months
and the second dose at 4 years.
c. On the grid below, graph the level of measles antibodies you would expect in a vaccinated child.
The child’s mother is known to have had measles as a child.
These are both really excellent questions. whys has given you an answer that is pretty close to the money, but I will clear up a couple of things.
whys is absolutely right in saying that immunity will theoretically drop to zero after some time. In the graph you've provided, it is dropping towards zero, but on both occasions a vaccine prevents it from doing so. This may help to explain why we vaccinate at these ages!
Theoretically, at the end of each year you are left with fewer B-cells specific to measles, so your immunity does wane over time. This is particularly the case for vaccine-induced immunity, which is far less robust than immunity that comes from having a disease.
Your second point is correct. The baby has immunity initially as it has acquired it from the mother. The antibodies cross the placenta and are with the baby from birth. Over time, these antibodies break down, so the baby gets more from the mother via breast milk. You suggested that the mother might not have immunity to measles, because it has been a long time since she has had the disease. whys was also unsure about whether the mother would actually have immunity in real life. The simple answer is that yes, she would! As I alluded to earlier, immunity acquired from getting a disease is a lot more robust (i.e. it lasts longer). So even though the mother had the disease as a child, you would expect that immunity to be with her until she dies.