How do vaccines train the immune system? Antibodies, memory cells and boosters

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A vaccine is a rehearsal. It shows the immune system what a particular germ looks like without making you ill with the disease, so that if the real thing turns up later your body recognises it and responds fast. Understanding how that rehearsal works also explains why some vaccines need more than one dose and why some are updated regularly.

The immune system in brief

Your body has two broad lines of defence.

Innate immunity is the fast, general response: inflammation, fever and cells that engulf anything that looks foreign. It acts within hours but does not learn.
Adaptive immunity is slower to start but specific. It learns to recognise particular invaders and remembers them. Vaccines work by training this second system.

Antigens: what the immune system recognises

The immune system does not recognise a whole virus or bacterium as such. It recognises particular molecular shapes on it, called antigens, often proteins on the germ's surface. A vaccine's job is to present one or more of these antigens safely. Vaccines do this in several ways:

Inactivated vaccines contain germs that have been killed.
Live attenuated vaccines contain weakened germs that can multiply a little but are not expected to cause the disease in healthy people.
Subunit, protein and conjugate vaccines contain only selected pieces of the germ.
mRNA vaccines deliver instructions that let your own cells make a harmless piece of the germ, such as a coronavirus spike protein, for a short time. The mRNA does not enter the cell nucleus, where DNA is kept, and it is broken down soon after use.
Viral vector vaccines use a harmless, modified virus to carry the instructions in.

What happens after the jab

Antigen-presenting cells pick up the antigen near the injection site and carry it to lymph nodes, where they display fragments of it.
Helper T cells that recognise those fragments become activated and help coordinate the response.
B cells that match the antigen multiply. Many become plasma cells that pour out antibodies, proteins that latch on to the germ, block it and mark it for destruction.
Killer T cells learn to recognise and destroy the body's own cells that have become infected.
Memory cells. A fraction of the activated B and T cells become long-lived memory cells. They are the real prize: if the genuine germ appears, they respond far faster and more strongly than the first time.

Feeling tired, achy or having a sore arm for a day or two afterwards is commonly the innate immune system reacting, and is usually short-lived.

Why some vaccines need boosters

The first exposure is a first lesson; repeats strengthen it.

Several doses in a schedule can produce a stronger, longer-lasting response than one dose.
Protection can wane. For some vaccines, antibody levels and protection fall over the years, so a booster reminds the immune system.
Germs change. Flu viruses change from season to season, so the vaccine is reformulated to match the strains expected to circulate. Some other vaccines are also updated as viruses evolve.
Different vaccines behave differently. Some give very long-lasting protection after a full course; others need regular top-ups. That is why schedules vary from vaccine to vaccine.

Protecting other people

When enough people in a community are immune, a germ struggles to find new people to infect and its spread slows or stops. This is often called herd or community immunity. It helps protect people who cannot be vaccinated, such as very young babies or some people with weakened immune systems. The proportion needed differs between diseases, depending on how easily each one spreads.

Questions about your own vaccinations

Which vaccines you need, and when, depends on your age, health, pregnancy, travel and where you live, and national schedules differ. This guide is general information, not personal medical advice. For your own situation, speak to your doctor, nurse or pharmacist, and use your national health service's official vaccination schedule. Have a question about how the immune system learns? Ask it in the comments.