No more than 450 milligrams (30 tablespoons) of blood per 8 weeks will be drawn from adult patients, and no more than 7 milliliters per kilogram of blood per 8 weeks will be drawn from patients under age 18. Local health care providers will refer patients to the study and will obtain the samples. Some patients may also be seen at the NIH Clinical Center. Those patients will have a full medical history and physical examination, along with chest X-ray, blood counts, blood chemistry, EBV serologies, and viral load. Other tests, such as CT scan or MRI, may be performed if medically indicated. Patients will be asked to undergo leukapheresis.