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How Autoimmune Disease Occurs

The Processes that Underlie these Self-destructive Disorders.

Jun 3, 2008

An increasing number of diseases are described as autoimmune. The processes behind these disorders are well understood but the triggers remain unclear.

You might be forgiven for thinking that doctors use this description when they don't really know the cause of the illness. However the reality is that although the precise trigger for autoimmune disease is unclear, a lot is known about the mechanisms involved. Interestingly, many autoimmune disease are more common in women than in men.

How the Immune System Works

The immune system provides the body's major defence against infection; without it none of us would survive long enough to reproduce. There are four major players involved in the immune response.

B lymphocytes: white blood cells which produce antibodies in response to antigens
T lymphocytes: white blood cells which control the B lymphocytes. T cells known as suppressor cells are key to autoimmunity.
Antibodies; proteins produced by B lymphocytes which destroy antigens.
Antigens: anything regarded as 'foreign' by the immune system, normally viruses and bacteria and bacterial toxins.

Autoimmunity

Other human cells are regarded as foreign and so are likely to be attacked by the immune system; this is the cause of rejection in transplants. Our bodies have mechanisms for recognising our own cells as friendly otherwise we would constantly be under attack from our own lymphocytes and antibodies.

How this recognition of friend and foe occurs is not precisely understood; it probably develops as the foetus matures in the womb, but it seems that the role of the T suppressor cell is crucial. These T cells keep the B lymphocytes in check. A military analogy would be that of a general keeping the troops disciplined and under control. It's been shown that low suppressor T cell activity correlates with increased incidence of autoimmune disease – there just aren't enough generals to keep the troops under control.

Another way autoimmune disease might develop involves the business of recognition of friend or foe. In many cases the immune system recognises foreign antigens by markers on the surface of the bacterium, virus or foreign cell. To keep to our military analogy, these markers are equivalent to friendly flags or enemy flags. The friendly flags would be markers on our own cells.

Sometimes the flags become unclear and the T cells can't tell which is friend or enemy. It's not quite understood why this happens. It is known that virus infection can alter the markers on our own cells. In fact this is one way in which viruses are dealt with. Once a virus infects a cell it can only be removed by the immune system destroying the cell, so it's necessary that the markers are altered to allow the cell to be attacked. Because of this it's possible that a persistent low-level viral infection could trigger an autoimmune disease.

Common Autoimmune Diseases

Autoimmune diseases differ according to which particular tissue is attacked by the immune system.

Thyroid Disease: Grave's disease occurs when the thyroid produces too much thyroid hormone. This is caused by antibodies forming against the receptors on thyroid cells.
Diabetes: the early onset form of this disease is caused by antibodies destroying the islet cells of the pancreas.
Ulcerative colitis: antibodies are produced which attack cells of the colon
Rheumatoid arthritis: antibodies form against joint membranes and other connective tissue.

Resources

Medicine: Eds. Souhami and Moxham. Pub. Churchill Livingstone 2002

An Introduction to General Pathology. Spector and Axford. Pub. Churchill Livingstone 1999.

The copyright of the article How Autoimmune Disease Occurs in Autoimmune Disease is owned by John Richard Roberts. Permission to republish How Autoimmune Disease Occurs in print or online must be granted by the author in writing.