But a team of researchers from Germany has found that in some instances antibodies produced by the immune system attack the tissue of the heart instead of carrying out their normal role of fighting infection.

The malfunction may be responsible for cases of congestive heart failure, a chronic condition in which the heart muscle becomes damaged.

Congestive heart failure can cause shortness of breath and extreme limitations in physical activity.

The researchers compared:

• People suffering from congestive heart failure caused by heart attack and coronary heart disease.
• People suffering from dilated cardiomyopathy - a form of congestive heart failure not linked to heart attack or coronary heart disease.
• Healthy people.

It was found that 26% of the 104 patients with dilated cardiomyopathy had auto-antibodies attacking a specific region of the heart called the beta-andrenergic receptors.

The auto-antibodies stimulated the heart to beat too rapidly.

In contrast, only 10% of people with the most common form of congestive heart failure, caused by heart attack and coronary heart disease, had auto-antibodies.

Only 1% of the 108 healthy individuals without congestive heart failure or other diseases had the auto-antibodies.

Beta blockers

The immune system attacks receptors in the heart

Dr Fritz Boege, head of the department of clinical chemistry at Medizinische Poliklinik, University of Würzburg, and co-author of the study, said this research might explain why drugs called beta-blockers are effective in many patients with congestive heart failure.

The beta-blockers act on the beta-andrenergic receptors to slow the heart's beating.

Dr Roland Jahns, of the department of cardiology at same clinic, said, "Individuals with heart failure make extra adrenaline, a hormone that helps the heart pump faster.

"The binding of adrenaline to the beta-receptors makes the heart beat even faster and contract more strongly.

"The auto-antibodies seem to prolong the active state of the receptors. They stimulate the receptor, which enhances the effect of adrenaline."

Researchers found that in dilated cardiomyopathy, the hearts of those who had the auto-antibodies pumped less blood compared to those who did not.

Dr Jahns said: "The constant beta-receptor activity leads to a vicious cycle of overdrive wearing out the heart muscle."

Test tube studies showed that adding the beta-blocker bisoprolol to the individual's blood could block the auto-antibody.

Further research

However, they add, it is too soon to conclude that there is a cause and effect relationship between auto-antibodies and heart failure.

And researchers do not understand yet why the auto-antibodies are produced.

Belinda Linden, a cardiac nurse adviser for the British Heart Foundation, said more research was needed before beta-blockers were widely prescribed to treat congestive heart failure.

She said beta-blockers had been found to have a negative impact on people suffering from the condition in the past.

The cause of many cases of congestive heart failure is unknown. The disease has been linked to alcoholism, viral infection and exposure to toxic substances. It can also sometimes develop during pregnancy.

Usual treatments are diuretics to reduce the accumulation of fluid in the lungs, and ACE inhibitors which dilate the blood vessels and improve the pumping action of the heart.