(NIAID via AP)

Allergy sufferers know what happens if they eat something or get bitten by something they're allergic to: rash, swelling and, in extreme cases, trouble breathing, shock, heart attack and death.

Women tend to have these life-threatening allergic reactions — known as anaphylaxis — more frequently than men, but scientists haven't been able to figure out why.

Now research from the National Institutes of of Allergy and Infectious Diseases suggests that estrogen, a female hormone, may play a key role.

Researchers found that female mice experience more severe and longer-lasting anaphylactic reactions than males. They found that estrogen enhances the levels and activity of an enzyme lining the blood vessels, which in turn causes some of the severe allergic reactions. The enzyme increases the production of a substance called nitric oxide, which creates a drop in blood pressure and allows fluid in blood vessels to leak into the tissues, resulting in swelling.

When the researchers blocked that enzyme's activity, the difference between the male and female mice disappeared.

More work is needed to see if effects are similar in people. But researchers say the study should raise awareness among several groups: health-care providers, women of child-bearing age, post-menopausal women taking hormone replacement therapy and women taking a popular athletic performance supplement called L-arginine, which increases the production of nitric oxide.

"More women than men are admitted to hospitals for anaphylaxis, and that tells you something is going on here," said Dean Metcalfe, chief of NIAID’s Laboratory of Allergic Diseases and an author of a study released Monday in The Journal of Allergy and Clinical Immunology.

"Too often these gender differences are not focused on," he said. "We need to be better at associating diseases with gender."

For example, if a woman starts to have these severe allergic reactions and is taking birth control during her reproductive years, a doctor needs to know that to rule out hormonal association, he said.  Also, "if a woman talks to me about having anaphylaxis, among the questions I would ask is, 'are you taking health food supplements?'" such as L-arginine, he said.

The standard approach to treating the condition is the administration of epinephrine. Many people prone to severe allergic reactions carry injectable epinephrine, such as an EpiPen.

Anaphylaxis is extremely difficult to study in humans because it's life-threatening. There are about 200 documented deaths due to anaphylaxis every year in the United States, he said. But the true number is likely higher because it's not known how many people die of anaphylaxis that goes unrecognized.

An older person with heart disease may die after a heart attack, he said. But the heart attack may have been triggered by a severe, whole-body allergic reaction.