Eosinophils are a type of white blood cell. Elevated eosinophil levels can damage the heart, nerves, and other organs, in the syndrome known as hypereosinophilic syndrome (HES). Some individuals have a hereditary form of HES known as familial eosinophilia (FE). More research on the causation and mechanisms of HES is needed in order to design more effective and less toxic therapies. This study will investigate FE and its genetic causes, damage mechanisms, and disease markers (such as blood test abnormalities). It will enroll approximately 50 individuals (both adults and children) from a previously studied family with FE. This is a long-term study of indefinite duration. Participants will undergo yearly clinical examinations including medical history, physical examination, bloodwork, EKG, echocardiogram, and pulmonary function tests, with additional or more frequent examinations and tests as required. In addition, participants will donate blood and tissue for research purposes. Both adult and child participants will donate blood. At the initial evaluation, adult participants will donate bone marrow. During the study, some adult participants will also undergo a limited number of leukaopheresis sessions, in which blood is donated from one arm, the blood is separated into red blood cells and other components, and the red blood cells are returned into the donor's other arm.