Mepolizumab in Episodic Angioedema With Eosinophilia

Background: Gleich syndrome is also called episodic angioedema with eosinophilia (EAE). People with EAE have episodes of swelling. They may also have itching, hives, fever, and weight gain. During episodes, the body has very high numbers of white blood cells, especially a kind called eosinophils. Researchers think a drug called mepolizumab could help. Objective: To see if mepolizumab causes EAE symptoms to be less severe and happen less often. Eligibility: People ages 18 or older with EAE. Design: Participants will be screened under NIH protocol 94-I-0079. Participants will have 8 visits over about 6 months. The timing of some visits will depend on each participant s EAE episodes. Visits will include: * Medical history * Physical exam * Blood and urine tests * Optional bone marrow collection at first or second visit. For this, a needle will be inserted through the participant s hip bone into the marrow. Participants will get mepolizumab 3 times over about 3 months. They will get their first dose when their eosinophils are at their lowest point. They will get the drug by IV. A needle will guide a thin plastic tube into an arm vein. The drug will be given through the tube over about 30 minutes. Participants will keep a daily online log for about 3 months. The log will track their weight, temperature, and EAE symptoms. During the whole study, they will complete 2 online questionnaires about their symptoms. They will fill out 1 daily and 1 monthly. Participants will have blood and urine tests 2-3 times a week. For these, they will go to their local doctor.