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Severe Combined Immunodeficiency

Severe combined immunodeficiency (SCID) is an inherited primary immunodeficiency disease (PIDD) that typically presents in infancy results in profound immune deficiency condition resulting in a weak immune system that is unable to fight off even mild infections. It is considered to be the most serious PIDD.

SCID is caused by genetic defects that affects the function of T cells. Depending on the type of SCID, B cells and NK cells can also be affected. These cells play important roles in helping the immune system battle bacteria, viruses and fungi that cause infections. Affected infants will often die within the first year of life without treatment with hematopoietic stem cell transplantation. (HSCT) Newborn screening for SCID is able to identify infants before they get sick, leading to a shorter time to transplant and offering improved outcomes following transplantation. Transplantation within the first 3 months of life offers the best chances for successful outcomes.

There are several forms of SCID. The most common type is linked to a problem in a gene on the X chromosome, affecting only males. Women may carry the condition, but they also inherit a normal X chromosome. Since starting newborn screening for SCID, recessive forms of the disease that can affect boys and girls have been identified with increased frequency.

Other forms of SCID are caused by a deficiency of the enzyme adenosine deaminase (ADA) and a variety of other genetic defects.

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