Immune system

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What is the immune system?

The immune system defends the body from infection. It contains a complex network of cells, chemicals, tissues and organs. An underactive or overactive immune system can cause health problems.

The immune system’s job is to protect the body from infection. It recognises ‘invaders’ such as bacteria, viruses and fungi and abnormal cells like cancer cells. It creates an immune response and helps the body fight the invasion.

When harmful germs enter and invade the body, the body produces white blood cells to fight the infection. The white blood cells identify the germ and produce antibodies to fight it. Antibodies are proteins produced by the immune system that fight germs that cause disease. White blood cells also help with other immune responses, and can ‘remember’ the attack they have launched.

‘Remembering’ the attack is important when the body is exposed to the same germ in the future. It is also important for understanding how vaccines work. Vaccines expose the immune system to a dead or weakened germ, or to proteins from a germ. The body can recognise and respond very quickly to any future exposure to the same germ.

What are the parts of my immune system?

The immune system involves many parts of your body. Each part plays a role in recognising germs, communicating with other body parts, and working to fight the infection. Parts of the immune system include your skin, bone marrow, thymus, lymphatic system, lymph nodes, spleen and mucous membranes.

How does my immune system work?

Each part of your immune system works in different ways.

Skin is the first line of defence, and helps keep germs out of your body.
Bone marrow helps produce immune cells.
The thymus is a gland in the upper chest where some immune cells mature.
The lymphatic system is a network of tiny vessels that allow immune cells to travel between tissues and the bloodstream. The lymphatic system contains lymphocytes (white blood cells; mostly T cells and B cells). Lymphocytes try to recognise any bacteria, viruses or other foreign substances in the body and fight them. They are carried in a milky fluid called lymph.
Lymph nodes are small lumps of body tissue in the groin, armpit, around the neck and elsewhere that help the lymphatic system to communicate. They can become swollen when the body has an immune response.
The spleen, is an organ under your ribs on your left side that processes information from the blood.
Mucous membranes, like the lining of the inside of your mouth and nose, trap germs and allow your immune cells to catch and attack them.

Illustration showing the various organs of the immune system.

What immune conditions are there?

There are many different immune conditions. Some are due to underactivity or overactivity of the immune system.

An overactive immune system is related to disorders such as allergies and autoimmune diseases:

Allergies involve an immune response to something considered harmless for most people, such as pollen or a certain food.
Autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis, happen when the immune system attacks normal, healthy parts of your body.

An underactive immune system, or immunodeficiency, can increase your risk of infection. If you have immunodeficiency, your body doesn’t fight infection as it should. You may be born with an immunodeficiency (known as primary immunodeficiency (PID), or get it from a medical treatment or another disease (known as secondary immunodeficiency).

What are some of the symptoms of immune conditions?

If you have an immunodeficiency, you may have some of the following symptoms:

poor growth (children)
recurrent or severe infections (pneumonia, sinusitis, skin abscesses, ear infections, meningitis)
infections with unusual germs
chronic diarrhoea
oral thrush
severe eczema

If you have allergies, you may have some of the following symptoms: