The Immune System

Meeting a germ can change how the immune system responds next time. During an infection, some immune cells learn to recognise features of that germ. Some cells remain afterwards and help the body remember it.

A later encounter with the matching germ can bring a faster response. This memory is useful, but it does not make illness impossible. Protection may weaken over time, and memory of one germ does not protect against every other germ.

Vaccination can prepare immune memory before an infection with the disease-causing germ. Vaccine ingredients vary. Some use weakened or inactive germs. Some use germ parts or instructions for making a harmless germ part. The immune system can learn to recognise what the vaccine presents.

The aim is to lower the risk of illness. Vaccination adds preparation to the body's existing defences; skin and white blood cells still have work to do.

A white blood cell is not a tiny person chasing germs. It is a living cell whose actions help defend the body. Different kinds do different jobs.

Some surround and swallow bacteria. Some produce antibodies, proteins that attach to particular targets and help the immune system respond. Other white blood cells can destroy body cells infected by viruses. Attacking an infected cell can help limit the virus, which needs living cells to multiply.

These cells do not work only inside blood vessels. They also act in body tissues. Their responses can continue while a person feels ill.

The name 'white blood cells' covers several kinds of helpers. It does not mean that every one of them can recognise and destroy every germ in the same way.

Think about three places where germs might reach your body: your hand, your eye and your nose. Each has ways to reduce the chance of germs reaching tissues inside.

Unbroken skin blocks many germs on your hand. The skin also sheds outer cells, carrying away some microbes attached to them. Tears help wash the surface of your eye. Sticky mucus inside your nose can trap germs and other particles.

These barriers do not block everything. A cut creates a gap in skin, and some germs can pass the defences around body openings. A cut does not always become infected, but it can provide a way in.

If germs enter tissues, white blood cells can help fight them. Blocking entry and responding inside the body are different defence jobs.

A germ does not have to be familiar before the body can respond to it. Many defences are available before an infection begins. Skin is already a barrier, and some white blood cells can respond to signs shared by groups of germs.

Scientists call these innate defences. They do not depend on a stored memory of one particular germ. For example, some white blood cells can swallow bacteria they have not met before.

The body also has an adaptive response. This part learns to recognise particular targets and can keep immune memory. It can develop during an infection, and vaccination can prepare it beforehand.

Innate and adaptive responses work together. Neither is a promise that every infection will be stopped. A person can become ill while the immune system is responding.

An injured area can look red or swollen even when no germs have entered. Injury and infection can both trigger inflammation, a response that brings changes to nearby tissue.

Signals from cells can increase local blood flow, making an area warmer and redder. Fluid moving into the tissue can cause swelling. Immune cells can enter the area to fight germs and help clear damaged cells. These changes may also make the area sore.

Some infections cause a fever. Chemical signals change how the body controls its temperature, raising it above its usual level. Fever can support immune defence, but it does not prove that an infection is harmless. Too much inflammation can damage tissue too.

Tell an adult if you have a fever, feel unwell, or a cut becomes more red, swollen or painful. A health professional can advise whether care is needed.