This article was last reviewed on
This article waslast modified on 7 March 2022.

What is the immune system?

The immune system is the body’s means of protection against "foreign" substances such as bacteria and viruses. It is composed of two major parts. One component involves proteins such as antibodies. These recognise "foreign" substances and cause them to be removed from the body. The other component involves specialised blood cells called T lymphocytes. These cells attack and destroy "foreign" substances directly. Antibodies and T lymphocytes become protective only after they are exposed to a "foreign" substance for the first time. This is the reason that we use vaccinations: to allow our immune system to recognise weakened or inactivated forms of bacteria and viruses that can cause disease. We will then be protected if we actually come in contact with them.

What are autoimmune disorders?
Normally the immune system recognises the tissues in the body that are not "foreign" and does not attack them. Autoimmune disorders are diseases caused by the body producing an immune response against its own tissues. The cause of autoimmune diseases is unknown, but it appears that there is a genetic predisposition to develop autoimmune disease in many cases (i.e. the risk is passed down through families). In a few types of autoimmune disease (e.g. rheumatic fever), a bacteria or virus triggers the immune system as normal but the antibodies or T-cells produced attack normal cells. This may happen because some part of these normal cells resembles a part of the infecting germ and so the immune system attacks the normal cell as though it is an invading germ.

Autoimmune disorders fall into two general types: those that damage many organs ('systemic'), and those where only a single organ or tissue is directly damaged by the autoimmune process ('localised' or ‘organ specific’). However, the distinctions can become blurred as the effect of an organ specific autoimmune disease sometimes extends beyond the affected organ, indirectly affecting other body organs and systems. Some of the most common types of autoimmune disorders include:


Systemic Autoimmune Diseases Localised Autoimmune Diseases
Rheumatoid arthritis (joints; less commonly lung, skin) Type 1 Diabetes Mellitus (pancreas islets)
Lupus [Systemic Lupus Erythematosus] (skin, joints, kidneys, heart, brain, red blood cells, other) Hashimoto thyroiditis, Graves disease (thyroid)
Scleroderma or systemic sclerosis (skin, intestine, less commonly lung)

Coeliac disease, Inflammatory bowel disease [Crohn disease, Ulcerative colitis] (GI tract)

Sjogren syndrome (salivary glands, tear glands, joints) Multiple sclerosis*, Guillain-Barre syndrome (brain)
Goodpasture syndrome [GBM disease] (lungs, kidneys) Addison disease (adrenal glands)
Granulomatous polyangiitis [Wegener granulomatosis] (sinuses, lungs, kidneys) Primary biliary sclerosis, Sclerosing cholangitis, Autoimmune hepatitis (liver)

* There is still some debate as to whether MS is an autoimmune disease

In some cases, a person may have more than one autoimmune disease; for example, people with Addison disease often have type 1 diabetes, while those with sclerosing cholangitis often have either ulcerative colitis or Crohn disease.