IGCSE Biology 0610: Booster Vaccination, Antibody Levels and Memory Cells

Ranjit receives his first vaccination against tetanus at age 5. His body responds by producing antibodies against the tetanus antigen; the level of these antibodies in his blood rises over the following weeks, then gradually falls over the following months and years. At age 15, Ranjit receives a booster vaccination containing the same tetanus antigen. This time, the level of antibodies in his blood rises much faster and reaches a much higher peak than it did after the first vaccination, and Ranjit does not become ill at any point.

(a) Explain, in terms of what happens inside Ranjit's body over time, why the level of antibodies after the first vaccination at age 5 does not stay high indefinitely. [2]

(b) Explain, in terms of memory cells, why the antibody response to the booster vaccination at age 15 is faster and reaches a higher level than the response to the first vaccination at age 5. [3]

(c) Explain why the protection Ranjit gains from the booster vaccination is described as active immunity, even though he received it by injection from a doctor rather than through natural infection. [2]

Part (a): Why the antibody level after the first vaccination falls over time

When Ranjit was first vaccinated at age 5, the tetanus antigen stimulated his lymphocytes to produce antibodies, and the level of these antibodies in his blood rose while this response was active. Once the antigen that triggered this response is no longer present in the body, the intense antibody-producing response comes to an end, and antibodies already in the blood are gradually broken down over time. This is why the antibody level rises and then falls, even though Ranjit’s protection is not entirely lost, the memory cells produced during this first response remain in his body.

Part (b): Why the booster response is faster and stronger

The first vaccination did more than cause a temporary rise in antibodies, it also caused memory cells specific to the tetanus antigen to be produced and to persist in Ranjit’s body for years afterwards.

When the booster vaccination introduces the same tetanus antigen again at age 15, these memory cells are able to recognise it immediately. This allows the relevant lymphocytes to start producing matching antibodies much more quickly, and in much larger numbers, than during the first vaccination, when no memory cells were yet present. This is why the antibody level after the booster rises faster and reaches a higher peak.

Part (c): Why the booster still counts as active immunity

The distinction between active and passive immunity depends on whose body actually makes the antibodies, not on how the antigen or antibodies reach the body.

The booster injection delivers the tetanus antigen, not ready-made antibodies. Once this antigen is inside Ranjit’s body, it is his own lymphocytes and memory cells that respond by producing his own antibodies against it. Because the antibodies protecting Ranjit are made by his own body, this is active immunity, even though a doctor administered the vaccination by injection rather than Ranjit catching tetanus naturally.

Final answers

(a) The antigen that triggered the response is no longer present, so the strong antibody-producing response ends and existing antibodies are gradually broken down, though memory cells remain.
(b) Memory cells produced after the first vaccination recognise the antigen immediately at the booster, allowing a faster, larger antibody response.
(c) Active immunity, because Ranjit’s own body makes the antibodies against the injected antigen. The immunity type depends on who makes the antibodies, not on how the antigen was delivered.