Immunity and vaccinationEdexcel A-Level Biology A: Revision notes
Section 1
Types of immunity
Immunity is the ability to resist a particular pathogen. It is classified in two ways.
- Active: the person's own immune system produces antibodies and memory cells.
- Passive: the person receives ready-made antibodies; their own immune system is not stimulated.
- Natural: acquired through normal events, such as infection or pregnancy.
- Artificial: acquired through medical intervention.
This gives four types: natural active, artificial active, natural passive and artificial passive.
Section 2
Active immunity: infection and vaccination
Natural active immunity follows an infection. The pathogen's antigens activate B and T cells, and after recovery memory cells remain.
Artificial active immunity follows vaccination. A vaccine contains antigens from the pathogen, such as a weakened (attenuated) or inactivated pathogen, a toxoid or a surface protein. The antigens stimulate the immune system to make plasma cells and memory cells without causing the disease. Booster doses give a larger and faster secondary response.
Active immunity takes days or weeks to develop but can last for years.
A vaccine does not contain antibodies. It contains antigens, and the person makes their own antibodies.
Section 3
Passive immunity
Natural passive immunity: antibodies pass from a mother to her baby across the placenta and in breast milk (colostrum).
Artificial passive immunity: antibodies are injected, for example antivenom after a snake bite or antibodies against rabies or tetanus.
Passive immunity works immediately because no response time is needed, but it is short-lived: the antibodies are proteins that are broken down, and no memory cells are formed.
Section 4
Primary and secondary responses
On first exposure to an antigen (the primary response), few B cells have the right receptor, so antibodies appear slowly, after a delay of several days, and the concentration is low.
On second exposure (the secondary response), memory cells are activated quickly and divide to form many plasma cells. Antibodies appear faster, in larger amounts and stay for longer. Worked example: a peak of 110 au compared with 20 au is times as high.
When explaining a faster second response, always mention memory cells and say that more cells are able to respond.
Section 5
Comparing the types
- Natural active: infection; own antibodies and memory cells; long lasting.
- Artificial active: vaccination; own antibodies and memory cells; long lasting.
- Natural passive: placenta or breast milk; no memory cells; short-lived.
- Artificial passive: injected antibodies; no memory cells; short-lived but immediate.
Use active immunity for long-term protection and passive immunity when immediate protection is needed, for example after a snake bite.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Immunity and vaccination
- Newborn babies have antibodies against measles in their blood at birth, received from their mothers across the placenta. The level of these antibodies falls to almost zero by about six months. In the UK the first measles vaccine dose is given at about twelve months.Explain why the antibodies received from the mother give the baby only temporary protection.2 marks
- A hospital records four cases. Person W recovers from chickenpox. Person X is given the hepatitis B vaccine. Person Y, bitten by a rabid dog, is injected with rabies antibodies. Person Z, a baby, receives antibodies from her mother's breast milk.Explain why the injection gives person Y protection straight away but only for a short time.2 marks
- A volunteer was given a vaccine on day 0 and a booster dose of the same vaccine on day 28. After the first dose the antibody concentration in the blood peaked at 20 arbitrary units (au) on day 14 and had fallen to 6 au by day 28. After the booster it peaked at 110 au on day 35 and was still above 40 au on day 120.Describe the differences between the antibody response to the first dose and the response to the booster dose.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).