How do vaccines work? The immune system and memory cells

We have probably all heard from older relatives that a child who has had chickenpox usually doesn’t catch it again. We also know that the vaccines we had as babies and at school age still protect us from some diseases years later. But how does the body “remember” a germ? And how does a vaccine create that memory? In this article we explain the basics of how the immune system works and how vaccines work, in language that is scientifically accurate but simple.

Note: this article is for general information. For personal health decisions and your vaccination schedule, always consult a doctor or your family health centre.

What is the immune system?

The immune system is a defence network of organs, cells and molecules that protects the body against disease-causing germs (bacteria, viruses, fungi and parasites). The bone marrow, lymph nodes, spleen and the white blood cells circulating in the blood are important parts of this system.

We can think of the immune system as two lines of defence:

1. Innate immunity: fast but not specialised. Skin and mucous membranes act like a wall. Stomach acid, some enzymes in tears and fever are also part of this defence. Some white blood cells (for example phagocytes) destroy foreign material that enters the body by engulfing it. The redness and swelling around a cut show this defence at work.

2. Acquired (adaptive) immunity: slower to start, but targeted and with a memory. There are two main players here:

• B lymphocytes: produce Y-shaped proteins called antibodies.
• T lymphocytes: some direct the immune response, while others destroy infected cells directly.

Antigen and antibody: a key and a lock

Every germ carries molecules on its surface that are specific to it. These molecules, which the immune system recognises as “foreign”, are called antigens.

Antibodies are proteins that fit a particular antigen the way a key fits a lock. When an antibody binds to its antigen, it can neutralise the germ, stop it from entering cells, or “tag” it for other immune cells.

Our body contains a huge variety of B and T lymphocytes, each able to recognise a different antigen. When a germ enters the body, the lymphocytes that recognise its antigen are activated, multiply quickly and join the fight.

Immune memory: why is the second time easier?

The first time we meet a germ, it can usually take from a few days to a few weeks for acquired immunity to come into play at full strength. During that time the germ multiplies and we get ill.

After the infection is over, some of the activated lymphocytes become memory cells. These cells can stay in the body for a long time: in some cases for years, or even for life. When the same germ comes back, the memory cells recognise it immediately. The response is much faster and stronger. Often the germ is defeated before we even notice we are ill.

How do vaccines work?

The basic idea of a vaccine is this: to show the immune system the germ’s “ID card” without making us go through the disease itself. That way the body builds memory cells without ever meeting the real germ.

Vaccines can be made in different ways:

• Inactivated (killed) vaccines: contain germs that have been killed. They cannot multiply, but their antigens are presented to the immune system.
• Live attenuated vaccines: contain germs whose ability to cause disease has been greatly reduced. The measles, mumps and rubella (MMR) vaccine is an example of this type.
• Subunit vaccines: contain only a specific part of the germ (for example a surface protein).
• Toxoid vaccines: contain a harmless form of the toxins produced by some bacteria. The tetanus vaccine is an example.
• mRNA vaccines: give our cells instructions to make a specific protein of the germ for a short time. The immune system learns to recognise this protein. The mRNA does not enter the cell nucleus and does not change our DNA; it is broken down within a short time.

Some vaccines need more than one dose, or a booster dose years later. This is because some immune responses weaken over time, or because the memory needs to be stimulated again to strengthen it.

A short note from history

At the end of the 18th century, the English doctor Edward Jenner noticed that milkmaids who had caught cowpox did not get smallpox, and in 1796 he carried out the first systematic vaccination. Before that, in the Ottoman Empire and elsewhere, a riskier practice against smallpox called “variolation” was already known. After intensive vaccination campaigns, the World Health Organization declared in 1980 that smallpox had been completely eradicated worldwide.

Herd immunity

When enough people in a community are immune to a disease, it becomes harder for the germ to spread from person to person. This also indirectly protects people who cannot be vaccinated, such as babies too young for a vaccine or patients with weakened immune systems. This is called community immunity (herd immunity). The proportion needed varies with the disease; for very contagious diseases such as measles it is very high.

Common misconceptions

“A vaccine gives you the disease it is meant to prevent.” Inactivated, subunit, toxoid and mRNA vaccines contain no germs that can multiply, and cannot cause the disease. After a vaccine you may get a mild fever, a sore arm or tiredness; these are mostly short-lived reactions showing that the immune system is working.

“Getting the disease naturally is always better.” Having the disease does give immunity, but the disease itself can lead to serious complications. Vaccines aim to give immunity without that risk.

“Antibiotics work against viruses too.” Antibiotics act on bacteria, not on viruses. Taking antibiotics for viral illnesses such as flu and the common cold doesn’t help and can contribute to antibiotic resistance.

“The harder the immune system works, the better.” What matters is that the immune system works in balance. Allergies are an excessive immune response to harmless substances.

Try it at home: glitter hands and the spread of germs

Germs are invisible to the eye, but you can watch them spread using a model.

You will need: a teaspoon of hand cream or a little cooking oil, a pinch of glitter, soap, water.

1. Rub a little cream onto one person’s palms and sprinkle glitter on top. The glitter represents “germs”.
2. This person shakes someone else’s hand, then holds a door handle and a pen.
3. Look at where, and onto whom, the glitter has spread.
4. Now wash your hands with water only; then wash them with soap for at least 20 seconds. Compare the difference.

This model shows how easily germs spread by contact and why washing your hands with soap matters. Remember: this is a model; real germs are much smaller than glitter, and some also spread through droplets in the air.

Summary

• The immune system has two lines of defence: innate and acquired.
• Antigens are a germ’s “ID card”; antibodies are proteins that bind specifically to them.
• Memory cells give a fast, strong response the second time we meet the same germ.
• Vaccines build immune memory without making us go through the disease.
• When enough people are immune, herd immunity also protects those who cannot be vaccinated.

For more about living things and our bodies, take a look at our Science topics and our biology articles.