RecruitingNatural History of Dysregulation and Aging of the Immune System in People With Trisomy 21 With and Without ThymectomyBackground: Down syndrome is a genetic disorder that can cause heart defects and other problems in the body. People with Down syndrome are more likely to have infections, autoimmunity, and blood diseases. Some may need surgery to treat congenital heart problems. During this surgery, doctors sometimes remove part of the thymus. The thymus is an organ that plays a role in immune function. People who have had part of their thymus removed may get sick more often than others do. Objective: This natural history study will gather data about how removing part of the thymus affects the health of people with Down syndrome. Eligibility: People aged 1 year and older with Down syndrome. The study will include both people who have, and those who have not had, surgery to remove part of their thymus. Healthy relatives are also needed. Design: Participants with Down syndrome will have clinic visits at least once a year for 15 years. At each visit they will have a physical exam. They will give blood and stool samples. They will have tests of their heart and lung function. Participants aged 18 years or older may have at least 1 imaging scan: They will lie on a table that slides into a donut-shaped machine. The machine uses X-rays to take pictures of the inside of the body. Participants who have tissue samples collected from their bodies (biopsies) taken during the study may have extra tissue taken for research. Healthy relatives will also have visits once a year for 15 years. They will only have a physical exam and provide blood and stool samples.