Immune System (Fighting Germs)

Your body is under constant attack. Tiny organisms are continually trying to get inside you and multiply, which can make you sick. Fortunately, your body has a powerful immune system to repel the invaders.

The first line of defense against germs is your body's surface, which acts as a barrier. The surface includes not only your skin but also the surface of your eyes and the soft tissues lining your mouth, nose, throat, and stomach. If germs find a break in any part of your body - such as a cut - the damaged tissue reacts immediately by becoming inflamed: it swells and fills with germ-destroying blood cells.

Many parts of the immune system work to block all kinds of germs, but others are more specific. Your adaptive immune system identifies new germs and then targets them specifically. It also remembers them for the future, giving you immunity to the diseases they cause.


Whenever you touch something or breathe in, you pick up tiny organisms too small to see. Most do no harm, but some try to invade your body and feed on you. Harmful microorganisms are called germs. The most common types are viruses and bacteria. Viruses cause colds, warts, and many diseases. Bacteria make wounds swell up and can also cause various diseases.

Bacteria: Bacteria are single-celled organisms. They are so tiny that hundreds could fit on the point of a needle.

Defense force

The human body contains about 50 billion white blood cells. These are the body's defenders. They seek out germs and kill them, using a variety of different methods. White blood cells called macrophages kill by swallowing germs whole and digesting them.

How antibodies work

Antibodies are chemicals that stick to specific kinds of germs, flagging them for destruction. There are millions of different germs, but the human body can manufacture 10 billion different antibodies, ensuring there's one for any germ you encounter. Once an antibody cell has been activated by meeting a matching germ, it makes copies of itself and makes the body immune.

A vaccine triggers your body into producing antibodies, making you immune to a disease without having to suffer it.

Filtering germs

Germs that break through the body’s barriers and invade internal tissues do not usually survive for long. The human body contains a network of tiny vessels that collect fluid from every organ and carefully filter it for germs, which are swiftly destroyed. This network of vessels is called the lymphatic system.


The largest organ in the lymphatic system, the spleen filters blood for germs, destroys antibody-coated germs, and serves as a store for blood cells.

Lymphatic vessels

The vessels of the lymphatic system run through every part of the body.


With every blink, watery tears wash dirt and bacteria off the surface of your eyes. Tears also contain lysozyme, a chemical that destroys the cell walls of bacteria.


Continually produced by glands in your cheeks and under your tongue, saliva flushes germs out of your mouth and into your stomach, where acid destroys them. Saliva also contains a range of antibacterial chemicals that attack germs.


These soft red areas at the back of the mouth are packed with white blood cells that destroy germs from food or the air. When you have a sore throat caused by viruses or bacteria, your tonsils swell up as they help fight the germs.


Your skin forms a thick, protective barrier that germs cannot cross, unless the skin gets cut. Glands in the skin secrete sweat and an oily fluid called sebum, both of which contain chemicals that repel germs.

Stomach acid

The lining of the stomach makes powerful hydrochloric acid, which destroys germs in food. It also kills the germs in mucus from the throat, which we swallow regularly to help keep the airways clean.

Lymph node

As body fluids flow through the vessels of the lymphatic system, they are filtered through swellings called lymph nodes, which vary from the size of a period to the size of a grape. They are packed with white blood cells that screen the passing fluid for germs and destroy them.