Severe combined immunodeficiency (SCID) is a rare genetic disease that is actually a group of inherited disorders characterized by a lack of immune response, usually occurring in infants less than six months old. SCID is the result of a combination of defects of both T-lymphocytes and B-lymphocytes. Lymphocytes are white blood cells that are made in bone marrow, and many move to the thymus gland where they become specialized immune T and B cells. In healthy individuals, T cells attack antigens while B cells make plasma cells that produce antibodies (immunoglobulins). However, this immune response in SCID patients is absent making them very susceptible to invading diseases, and thus children with untreated SCID rarely live to the age of two years.

SCID is characterized by three main features. The helper T-lymphocytes are functioning poorly or are absent, the thymus gland may be small and...