Active and passive immunity, vaccination and herd immunity - TCE Biology (Tasmania)
“Vaccination and the concept of herd immunity: analyse the similarities and differences between passive immunity (for example antibodies acquired via the placenta or via antibody serum injection) and active immunity (acquired through the actions of the immune system as a result of natural exposure to a pathogen or through the use of vaccines)”
Active immunity is made by your own immune system after exposure to an antigen, either naturally (catching the disease) or artificially (a vaccine); it produces memory cells, so it is long-lasting but takes time to develop. Passive immunity is receiving ready-made antibodies, either naturally (across the placenta or in breast milk) or artificially (an injection of antibodies such as antivenom); it works immediately but is short-lived because no memory cells are made. Vaccination produces artificial active immunity, and when enough of a population is immune, herd immunity protects those who are not.
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What this dot point is asking
Types of immunity are part of criterion 7 (Section D of the TASC exam). The external assessment specifications include naturally and artificially acquired active and passive immunity, and an understanding of vaccination and herd immunity.
Four types of acquired immunity
| Active (own immune system makes antibodies and memory cells) | Passive (ready-made antibodies received) | |
|---|---|---|
| Natural | Recovering from an infection, such as chickenpox | Antibodies crossing the placenta to the fetus, or in breast milk |
| Artificial | Vaccination | Injection of antibody serum, such as antivenom after a snakebite |
Comparing active and passive immunity
| Active immunity | Passive immunity | |
|---|---|---|
| Source of antibodies | The person's own plasma cells | Another person or animal, or a laboratory |
| Memory cells | Yes | No |
| Speed | Takes days to weeks to develop | Immediate protection |
| Duration | Long-lasting (years, sometimes life) | Short-lived (weeks to months), as antibodies are broken down |
| Use | Long-term protection | Immediate protection, for example after exposure to venom or for newborns |
Only active immunity produces memory cells, so only active immunity gives long-term protection. Passive immunity fades as the borrowed antibodies are broken down.
How vaccines work
A vaccine contains antigens from a pathogen in a form that cannot cause the disease, such as an inactivated (killed) or weakened pathogen, purified parts of it, an inactivated toxin, or genetic instructions for making a pathogen protein.
- The antigens are taken up by antigen-presenting cells.
- Helper T cells, B cells and cytotoxic T cells specific to the antigen are activated (a primary response).
- Memory B and T cells are formed.
- If the person is later exposed to the real pathogen, the memory cells produce a rapid, strong secondary response and the pathogen is destroyed before it causes disease.
Booster doses re-expose the immune system to the antigen, increasing the number of memory cells and the level of antibodies.
Herd immunity
Herd immunity is the indirect protection of people who are not immune (babies too young to be vaccinated, people with weakened immune systems) when a large enough proportion of the population is immune. With few susceptible hosts, each infected person passes the pathogen to fewer others, so chains of transmission break and outbreaks die out. The proportion needed depends on how easily the pathogen spreads: highly contagious diseases such as measles require very high vaccination coverage. If vaccination rates fall below that level, outbreaks can return.
Whooping cough (pertussis)
Pregnant women in Australia are offered a whooping cough vaccine during pregnancy.
- The mother develops artificial active immunity and makes antibodies.
- Her antibodies cross the placenta, giving the baby natural passive immunity for its first weeks, before it can be vaccinated.
- The baby's protection is temporary because it has no memory cells of its own; it needs its own vaccinations later for artificial active immunity.
- Herd immunity from high vaccination rates in family members and the community further reduces the baby's exposure.
Marker's note: classify each step and explain whether memory cells are involved.
- Calling a vaccine passive immunity
- A vaccine stimulates the person's own immune system: artificial active immunity.
- Saying antibodies from the mother are active immunity for the baby
- The baby did not make them: natural passive immunity.
- Saying herd immunity makes unvaccinated people immune
- They remain susceptible; they are protected because the pathogen cannot spread easily.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original4 marksClassify each of the following as natural or artificial and active or passive immunity: (a) recovering from measles; (b) receiving a tetanus vaccine; (c) a baby receiving antibodies in breast milk; (d) receiving antivenom after a snakebite.Show worked answer →
One mark each.
(a) Natural active immunity: the person's own immune system responded to the pathogen.
(b) Artificial active immunity: the vaccine stimulated the person's own immune system to make antibodies and memory cells.
(c) Natural passive immunity: ready-made antibodies came from the mother.
(d) Artificial passive immunity: ready-made antibodies were injected.
Original5 marksExplain how vaccination of a large proportion of a population can protect a newborn baby who is too young to be vaccinated.Show worked answer →
Vaccination (2 marks). A vaccine contains antigens from a pathogen (for example inactivated or weakened pathogen, or parts of it) that stimulate a primary immune response and the formation of memory cells, so vaccinated people can quickly destroy the real pathogen and are unlikely to become infected or pass it on.
Herd immunity (3 marks). When a high proportion of the population is immune, the pathogen has few susceptible hosts, so chains of transmission are broken and the pathogen cannot spread widely. The newborn is therefore unlikely to come into contact with an infectious person, even though the baby itself has no memory cells. This protection depends on vaccination rates staying above the level needed for that disease.