IGCSE Biology 0610: Vaccination, Antigens and Active Immunity

A university health clinic is trialling a new nasal-spray vaccine against one particular strain of influenza (flu) virus, called strain P. Volunteers are given the nasal spray, which contains a weakened form of strain P.

Several months later, some of the vaccinated volunteers are exposed to strain P and do not become ill. However, other volunteers are separately exposed to a different strain of flu virus, strain Q, whose antigens have a different shape from those of strain P. Several of these volunteers do become ill with strain Q, even though they were vaccinated against strain P.

(a) Explain what is meant by active immunity, referring to the nasal-spray vaccine in your answer. [2]

(b) Explain, in terms of antigens and antibodies, why volunteers who were protected against strain P were not protected against strain Q. [3]

(c) Outline the sequence of events that occurs inside a volunteer's body, from the moment the nasal-spray vaccine is given, that leads to long-term protection against strain P. [4]

Part (a): Active immunity and the vaccine

Active immunity is defence against a pathogen by the body making its own antibodies, rather than receiving ready-made ones.

The nasal-spray vaccine introduces a weakened form of strain P into a volunteer’s body. This is not enough to cause serious illness, but it is enough to trigger the volunteer’s immune system to produce antibodies against strain P. Because these antibodies are made by the volunteer’s own body, this is active immunity.

Part (b): Why protection against strain P does not cover strain Q

Every strain of flu virus carries its own antigens, and different strains have antigens with different shapes.

When a volunteer is vaccinated against strain P, their lymphocytes are stimulated by strain P’s antigens and produce antibodies with a shape that is complementary to those specific antigens.

Strain Q’s antigens have a different shape from strain P’s antigens. The antibodies the volunteer already has, which were shaped to fit strain P’s antigens, cannot bind effectively to strain Q’s differently-shaped antigens. Since the antibodies cannot bind to strain Q, they cannot bring about its destruction or mark it for phagocytes to engulf. As a result, being immune to strain P gives no protection against strain Q.

Part (c): From vaccination to long-term protection against strain P

The events that lead to long-term protection happen in a clear sequence:

The nasal spray introduces the weakened strain-P virus (or its antigens) into the volunteer’s body.
These antigens stimulate lymphocytes, which respond by producing antibodies that fit strain P’s antigens.
The antibodies bind to strain P’s antigens, either destroying the weakened virus directly or marking it so that phagocytes can destroy it.
Alongside this response, memory cells are produced. These persist in the body long after the initial antibody levels have fallen, and allow a much faster and larger antibody response if the volunteer meets strain P again. This is what gives long-term immunity.

Final answers

(a) Active immunity: the volunteer’s own body makes antibodies against strain P after the vaccine introduces a weakened form of the virus.
(b) Strain Q’s antigens have a different shape from strain P’s antigens, so the antibodies made against strain P do not fit (cannot bind to) strain Q’s antigens.
(c) Weakened strain P introduced → antigens stimulate lymphocytes → antibodies produced and bind to/mark strain P → memory cells produced, giving long-term protection.