Immune Suppression - Immune system

What is the immune system?

We are surrounded by millions of bacteria, viruses and other germs (microbes) that have the potential to enter our bodies and cause harm. The immune system is the body's defence against disease-causing microbes (pathogens).

The immune system is made up of non-specialised defences such as skin and the acidic juice produced by your stomach. However, it also has some highly specialised defences which give you resistance to particular pathogens. Another name for this resistance is immunity. These defences are special white blood cells called lymphocytes. Other types of white blood cells play an important part in defending your body against infection.

The lymphatic system is also part of the immune system. The lymphatic system is made up of a network of tubes (vessels) which carry fluid called lymph. It contains specialised lymph tissue and all the structures dedicated to the production of lymphocytes.

Where is the immune system found?

The immune system is generally divided into two parts. The first part is the defences you are born with. These form what are known as the innate system. The second part of your immune system, known as immunity, develops as you grow. Your immunity gives you protection against specific pathogens. Not only can this system recognise particular pathogens, it also has a memory of this. This means that if you encounter a certain pathogen twice, your immune system recognises it the second time around. This usually means your body responds quicker to fight off the infection.

The innate system is found in many different places around the body.

  • First line of defence is your skin. Skin forms a waterproof barrier that prevents pathogens from entering the body.
  • Your body cavities, such as your nose and mouth, are lined with mucous membranes. Mucous membranes produce sticky mucus which can trap bacteria and other pathogens.
  • Gastric juice produced by your stomach has high acidity which helps to kill off many of the bacteria in food.
  • Saliva washes pathogens off your teeth and helps to reduce the amount of bacteria and other pathogens in your mouth.

If bacteria or other pathogens manage to get through these first-line defences, they encounter a second line of defence. Most of these defences are present in your blood, either as specialised white blood cells or as chemicals released by your cells and tissues.

The second part of your immune system, the part that gives you immunity, involves the activation of lymphocytes. Lymphocytes are found in your blood and also in specialised lymph tissue such as lymph nodes, your spleen and your thymus.

lymphatic system

See also separate leaflet called The Immune System for more information.

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Dr Mary Harding
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
29417 (v1)
Last Checked:
05 July 2017
Next Review:
04 July 2020

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.