Immunity — Cambridge IGCSE Biology
Antigens, antibodies, active immunity, vaccination and passive immunity.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Active immunity is defence against a pathogen by antibody production in the body, gained after an infection or by vaccination. Antibodies are proteins made by lymphocytes, each with a shape complementary to one specific antigen on a pathogen. Antibodies bind to antigens, which leads to direct destruction of the pathogen or marks it for phagocytes to destroy.
Vaccination is putting weakened pathogens or their antigens into the body to give active immunity without suffering the disease. The antigens stimulate lymphocytes to produce antibodies, and memory cells are made that give long-term immunity. When a large proportion of the population is vaccinated, the pathogen has fewer susceptible hosts, so the disease spreads less.
Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual. A baby receives antibodies from its mother across the placenta before birth and in breast milk after birth. No memory cells are produced and the antibodies are broken down, so the protection does not last.
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Quick check questions
What is an antibody?
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A protein that binds to a specific antigen.
What gives long-term immunity after vaccination?
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Memory cells.
Name two ways a baby gets antibodies from its mother.
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Across the placenta and in breast milk.
Why is passive immunity short-term?
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No memory cells are produced.
Frequently asked questions
How do vaccines work?
Vaccines work by putting weakened pathogens or their antigens into the body. The antigens stimulate lymphocytes to produce antibodies, and memory cells are made that give long-term immunity. If the real pathogen enters later, the memory cells respond quickly and make large amounts of antibody. Vaccines contain antigens, not antibodies.
Why do pathogens have antigens?
Antigens are molecules on the surface of every pathogen, and each pathogen has its own antigens with specific shapes. The body's lymphocytes make antibodies with a shape complementary to one specific antigen, so the antibody fits that antigen and no others. This is why a different antibody is needed for each pathogen.
What is the difference between active and passive immunity?
In active immunity, a person's own lymphocytes make antibodies after an infection or vaccination, and memory cells give long-term protection. In passive immunity, antibodies are acquired from another individual, such as across the placenta or in breast milk. No memory cells are produced, so passive immunity is only short-term.
Why is breast-feeding important for a baby's immunity?
Breast-feeding is important because breast milk contains antibodies from the mother, giving the baby passive immunity. The baby's immune system is not yet fully developed, so these antibodies protect it against pathogens, for example in the gut. The protection is short-term, because no memory cells are made and the antibodies are broken down.
How does vaccinating many people protect the whole population?
When a large proportion of the population is vaccinated, the pathogen has fewer susceptible hosts to infect. It is passed on less often, so the disease spreads less. This also protects people who cannot be vaccinated, because they are less likely to come into contact with an infected person.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026