Introduction

Letter from Maureen McGovern

Questions and Answers

The Immune System

Diagnosis and Treatment

MDA's Search for a Cause, Treatment and Cure

THE IMMUNE SYSTEM IN PM AND DM

The immune system is a group of organs and cells located throughout the body. Its members include the thymus, spleen, bone marrow and circulating white blood cells. When the immune system detects foreign substances, such as viruses or bacteria, the immune system attacks the invaders. Some white blood cells attack directly; others release proteins called antibodies, which attack the outsider.

In normal human development, self-reactive immune cells are destroyed, keeping the immune system from attacking the body's own tissues. But, researchers think, in some people, a few of these autoimmune cells may survive, setting the stage for an autoimmune disease such as PM or DM.

Some 30% of people with PM or DM have antibodies against Jo-1, a form of an important cell enzyme.

Another theory of autoimmunity is that normally protective cells may mistakenly attack healthy cells that look similar to those of outside invaders. There is also some evidence that genetic factors can make a person more likely to develop one or more autoimmune diseases. These factors may combine to contribute to the development of autoimmune diseases such as PM and DM.

In polymyositis, the muscle damage is caused principally when white blood cells called T cells directly attack the muscle fiber. In dermatomyositis, antibodies attack the capillaries within the muscle. The resulting loss of blood supply then damages the muscle cells.

Almost all people with PM or DM have detectable levels of self-reactive antibodies in their bloodstreams. About three in 10 carry autoantibodies to an important cellular protein called amino-acyl tRNA synthetase, and in particular to one form of it called Jo-1. The presence of antibodies against Jo-1 is correlated with a high rate of interstitial lung disease and other inflammatory conditions. In some cases, a rise in antibody levels has been observed before a period of worsening symptoms. Malignancy is less commonly found in patients with these antibodies.

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