From the article:
In the current study, NIAID researchers found that female mice experience more severe and longer lasting anaphylactic reactions than males. Instead of targeting immune cells, estrogen influences blood vessels, enhancing the levels and activity of endothelial nitric oxide synthase (eNOS), an enzyme that causes some of the symptoms of anaphylaxis. When the researchers blocked eNOS activity, the gender disparity disappeared. In addition, giving estrogen-blocking treatments to female mice reduced the severity of their allergic responses to a level similar to those seen in males.