This medication is used to treat an acute attack of a certain immune disease passed down through families (hereditary angioedema-HAE). Ecallantide blocks a natural substance produced by the body (kallikrein) which acts to increase amounts of another natural substance (bradykinin) thought to cause the symptoms seen during an acute attack of HAE. This medication does not cure HAE. Ecallantide may lessen the symptoms during an acute attack of HAE such as rapid swelling and pain of the hands, feet, limbs, face, tongue, or throat. When attacks involve the intestines, symptoms may include abdominal pain/cramps, diarrhea or vomiting. Attacks of swelling may occur without reason. However, anxiety, stress, sickness, and surgery may trigger attacks in some people.