A baby is protected by its mother's antibodies for a few months; a vaccinated adult may be protected for life. The difference is whether your own lymphocytes did the work and left memory cells behind.
Active immunity develops when your own lymphocytes respond to an antigen, making plasma cells, antibodies and memory cells: it takes time but is long-lasting. Passive immunity is gained when antibodies made by another organism are received: it is immediate but short-lived, because the antibodies are broken down and no memory cells form. Immunity is natural when it comes from normal life (an infection, antibodies across the placenta or in breast milk) and artificial when it comes from medical intervention (a vaccine, an injection of antibodies). A vaccine contains antigens, as dead or weakened pathogens, toxoids, purified proteins or mRNA, that stimulate a primary response and memory cells without causing disease, giving long-term immunity. Vaccination programmes control the spread of disease because when a high proportion of people are immune, the pathogen rarely reaches a susceptible person: herd immunity protects even those who cannot be vaccinated.
Fig. 1: the four types of immunity
Section 01
Active or passive, natural or artificial.
Click each of the four immunity chips and read what gives the protection. Then set the Vaccinated slider to 0% and press Introduce one infected person; repeat at 50% and at 80% or more. Compare the INFECTED count each time (green = vaccinated, red = infected, blue = recovered, grey = susceptible).
Active immunity
Passive immunity
source of antibodies
your own plasma cells, after an immune response to an antigen
made by another organism and given to you
memory cells
made
not made
speed
takes days to weeks (primary response)
immediate
how long it lasts
long-term, often years
short-term: weeks to months, as the antibodies are broken down
natural example
recovering from an infection
antibodies crossing the placenta, or in colostrum and breast milk
artificial example
vaccination
an injection of antibodies, for example antitoxin for tetanus or antivenom for a snake bite
Examiner trap
Passive immunity is short-lived because no memory cells are made, not because the antibodies are "weaker". Always mention memory cells when you compare active and passive immunity.
Stage 1 · Learn
Check what the sim just showed you
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In the sim, which type of immunity is described as antibodies crossing the placenta or coming in breast milk?
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In the sim, why is the passive immunity described as short-lived?
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In the sim, with the Vaccinated slider at 0%, what happens after one infected person is introduced?
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In the sim, why does the outbreak die out quickly when 80% or more are vaccinated?
Section 02
What is in a vaccine.
Every vaccine works the same way: it shows the immune system the antigens of a pathogen without letting the pathogen cause disease.
Contents: antigens in a safe form: live attenuated (weakened) pathogens, killed (inactivated) pathogens, toxoids (inactivated toxins), purified antigens such as surface proteins, or mRNA that body cells use to make the antigen for a short time.
Primary response: macrophages present the antigen, the complementary T-helper and B cells are selected and cloned, plasma cells make antibodies and memory cells form.
Long-term immunity: if the real pathogen enters later, memory cells give a fast secondary response, so it is destroyed before symptoms develop.
Boosters: a second or third dose triggers a secondary response, raising the number of memory cells and antibody level further.
Section 03
Vaccinating a population.
One vaccinated person is protected; a vaccinated population stops the pathogen spreading at all.
Herd immunity: when a high enough proportion of a population is immune, an infected person is unlikely to meet a susceptible one, so transmission chains break. People who cannot be vaccinated, such as newborn babies and people with weak immune systems, are protected too.
Mass vaccination: vaccinating most of the population, often as young children, keeps the proportion immune high as new people are born.
Ring vaccination: when a case is found, everyone around the case (contacts and the local area) is vaccinated, containing the outbreak. This helped to eradicate smallpox (declared in 1980).
Why it does not always work: some pathogens change their antigens (antigenic variation, as in influenza), so new vaccines are needed; some hide inside body cells or the gut, out of reach of antibodies; vaccines can be hard to store and deliver; and coverage falls if people refuse vaccination.
Examiner trap
Herd immunity does not mean every person is immune. It means enough people are immune that the pathogen cannot keep spreading.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
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A person bitten by a venomous snake is given an injection of antivenom antibodies. What type of immunity is this?
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Why does a vaccine give long-term immunity?
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Why do people need a new influenza vaccine most years?
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Which statement best explains how a vaccination programme can protect babies too young to be vaccinated?
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