What is the immune system?

The immune system comprises:

• skin
• bone marrow
• the thymus, a gland in your upper chest
• white blood cells, which fight infection
• lymph, a milky fluid carrying white blood cells
• the lymphatic system, a network of tiny vessels that carry lymph around the body
• lymph nodes, small lumps in your groin, armpit, around your neck and elsewhere
• the spleen, an organ under your ribs on the left
• mucous membranes, like the lining of the inside of your mouth.

The lymphatic system 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 your body and fight them.

Lymph nodes are found in certain areas such as the base of the neck and the armpit. They become swollen or enlarged in response to an infection.

How does the immune system work?

The skin and mucous membranes are the first line of defence against bacteria, viruses and other foreign substances. They act as a physical barrier, and they also contain immune cells.

When your skin has a cut, microbes can enter and invade your body. The cut triggers certain immune cells in the bloodstream that try to destroy the invaders.

In an infection, white blood cells identify the microbe, produce antibodies to fight the infection, and help other immune responses to occur. They also 'remember' the attack.

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

Related conditions

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 your immune system attacks normal components of the body.

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