What is the immune system?

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

The immune system’s job is to protect the body from infection. It recognises invaders such as bacteria, viruses and fungi as well as abnormal cells. It mounts an immune response to help the body fight the invasion.

When harmful microbes (tiny particles) enter and invade the body, the body produces white blood cells to fight the infection. The white blood cells identify the microbe, produce antibodies to fight it, and help other immune responses to occur. They also 'remember' the attack.

This is how vaccinations work. Vaccines expose the immune system to a dead or weakened microbe or to proteins from a microbe, so that the body is able to recognise and respond very quickly to any future exposure to the same microbe.

How does the immune system work?

The immune system involves many parts of the body. Each part plays a role in recognising foreign microbes, communicating with other parts of the body, and working to fight the infection. Parts of the immune system are:

• skin – the first line of defence
• bone marrow – helps produce immune cells
• the thymus, a gland in the upper chest where some immune cells mature
• lymphatic system, a network of tiny vessels which allows immune cells to travel between tissues and the bloodstream. The lymphatic system contains lymphocytes (white blood cells; mostly T cells and B cells), which 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, small lumps in the groin, armpit, around the neck and elsewhere that help the lymphatic system to communicate. They can become swollen when the body mounts an immune response
• the spleen, an organ under the ribs on the left that processes information from the blood
• mucous membranes, like the lining of the inside of the mouth

How can the immune system go wrong?

Overactivity of the immune system is related to disorders such as allergies and autoimmune diseases.

Allergies involve an immune response to something considered harmless in most people, such as pollen or a certain food.

Autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis, occur when the immune system attacks normal components of the body.

Underactivity of the immune system, or immunodeficiency, can increase the risk of infection. You may be born with an immunodeficiency, or acquire it due to medical treatment or another disease.