Sex hormones apparently have an effect on the immune system, perhaps explaining why women are four times as likely as men to develop arthritis and other so-called autoimmune diseases.

In a study on mice, researchers found that the female hormone estrogen weakens the body's suppressor cells, which keep the immune system in check and prevent it from attacking the body.

The male hormone testosterone, on the other hand, seemed to boost the action of suppressor cells.

"The point of this is that the immune system does not sit in a kind of splendid isolation," says Dr. Norman Talal, a researcher at the University of Texas Health Science Center at San Antonio. "It's very intertwined in the other systems of the body."

Arthritis is one of several diseases resulting from the immune system attacking the body, Talal writes in the Journal of Immunology. Another, systemic lupus erythematosus, was found to be made worse by the female hormone and better by the male hormone, Talal and his colleagues report.

Attempts to use male hormones to treat lupus and related diseases have been complicated by unwanted side effects, such as growth of facial hair, Talal says.

He suggests new drugs could be developed "that have the immune system effects without the serious side effects."