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.

In addition to frequent and often very severe respiratory infections, other symptoms of SCID in infants include poor growth, rashes that look like eczema, chronic diarrhea and recurrent thrush in the mouth, although all of these symptoms may not be present.

Often, SCID is associated with recurrent viral infections and causes several hospitalizations before it is discovered.

One unusual infection that can be present with SCID early on is pneumocystis pneumonia. Presence of this infection is a red flag for the need to evaluate the immune system for SCID.

Early detection of SCID is extremely important. Treating the disease in the first months of life offers a very positive success rate in helping to combat SCID.

As with other PIDD, screening tests can measure blood lymphocytes levels. A diagnosis of SCID can be made in utero, which is especially helpful if there is a family history of immunodeficiency diseases.

The only cure currently and routinely available for SCID is bone marrow transplant, which provides a new immune system to the patient. Gene therapy treatment of SCID has also been successful in clinical trials, but not without complications.

Research on gene therapy for SCID is continuing and may one day be a good option. If you think your child may have SCID, prompt evaluation by an immunology specialist is crucial for early treatment.

Other forms of combined immunodeficiency can occur that may not require bone marrow transplantation.