AS & A Level Biology 9700: Active, Passive Immunity, Vaccination and Herd Immunity

A new viral disease is spreading in a country. Public health officials introduce a vaccination programme, and doctors also give some newborn babies antibodies against the virus obtained from their mother's breast milk.

(a) State the type of immunity a baby gains from antibodies in its mother's breast milk. In your answer, use both of the terms active/passive and natural/artificial, with a brief justification for each. [2]

(b) Explain how vaccinating an older child against the virus can give them long-term protection, referring to memory cells in your answer. [3]

(c) Explain why the immunity gained from breast milk in (a) does not give the baby long-term protection, unlike the immunity gained from vaccination in (b). [2]

(d) After several years, a high percentage of the country's population has been vaccinated against the virus. Unvaccinated individuals in this population also become much less likely to catch the virus. Explain this observation. [3]

Part (a): Type of immunity from breast milk

The baby receives ready-made antibodies from its mother rather than making its own antibodies in response to the antigen, so this is passive immunity. The antibodies are obtained through a normal biological process, breastfeeding, rather than through a deliberate medical procedure, so this is also natural immunity. Overall, the baby has gained natural passive immunity.

Part (b): How vaccination gives long-term protection

A vaccine contains a harmless version of the virus’s antigen (for example, an inactivated form of the virus, or an isolated antigen from it), rather than the antibodies themselves. Introducing this antigen triggers a primary immune response: B-lymphocytes with a complementary antigen receptor are selected, and they divide and differentiate into plasma cells, which secrete antibodies against the antigen, and into memory cells.

The memory cells remain in the body for a long time after vaccination. If the vaccinated child is later exposed to the real virus, these memory cells allow a rapid secondary immune response: a large amount of antibody is produced very quickly, usually fast enough to destroy the virus before it can multiply enough to cause noticeable disease. Because this protection comes from the child’s own immune system and its memory cells, it is active immunity and lasts for a long time, often years.

Part (c): Why passive immunity from breast milk is short-lived

The antibodies the baby receives in breast milk were produced by the mother’s immune system, not the baby’s own B-lymphocytes. Because the baby’s B-lymphocytes were never activated by the antigen, no memory cells specific to the virus are formed in the baby.

The antibodies that were passed on are gradually broken down and removed from the baby’s blood over the following weeks. Once they have gone, the baby has no memory cells left to allow a fast secondary response, so the protection disappears. This is why passive immunity, unlike active immunity from vaccination, does not last long.

Part (d): Herd immunity

When a very high proportion of the population has been vaccinated (and so is immune), the virus has relatively few remaining susceptible hosts to infect. This makes it much harder for the virus to pass from one person to the next, breaking the chains of transmission through the population.

As a result, an unvaccinated person is much less likely to ever come into contact with someone who is currently infected, since infections are far less common and spread far less easily overall. This indirect protection of unvaccinated individuals, arising from a high level of immunity in the surrounding population, is called herd immunity.

Final answers

(a) Natural passive immunity. Passive because the antibodies are ready-made, not produced by the baby; natural because they are gained through breastfeeding, not medical intervention.
(b) The vaccine’s antigen triggers a primary response and the formation of memory cells, allowing a fast secondary response (long-term active immunity) if the real virus is met later.
(c) No memory cells are formed in the baby, and the given antibodies are broken down over time, so protection is only temporary.
(d) High vaccination coverage leaves the virus few susceptible hosts, reducing transmission and so indirectly protecting unvaccinated individuals (herd immunity).