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Primary Immune Deficiency Diseases

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Severe Combined Immunodeficiency (SCID)

SCID is a syndrome caused by mutations in at least 13 different genes. In this condition, there is a complete absence of B- and T-cell function. Until approximately 30 years ago, SCID was fatal within the first year or two of life. It is inherited as either an X-linked (disease caused by genes located on the X chromosome) or an autosomal recessive (disease caused when two copies of the abnormal gene are present) defect.


The most common form of the condition is X-linked, or caused by mutations in a gene on the X chromosome. Boys with this type of SCID have lymphocytes that cannot grow, develop, or communicate with B cells. As a consequence, they have low numbers of T cells or natural killer cells and their B cells do not function.

The best-known form of autosomal recessive SCID is caused by adenosine deaminase (ADA) deficiency, in which infants lack the ADA enzyme necessary for T-cell survival.

Signs and symptoms

Typically, symptoms occur in infancy and include serious or life-threatening infections, especially viral infections, which result in pneumonia and chronic diarrhea. Candida (yeast) infections of the mouth and diaper area and a type of pneumonia called Pneumocystis jiroveci also are common in affected infants.


A positive diagnosis for SCID involves confirming the following in an affected patient:

  • Low T- and B-cell blood counts
  • Recurrent infections
  • Very low levels of immunoglobulin (antibodies)

A doctor should test for SCID at birth if a person has an affected sister, brother, or other relative with SCID. To help diagnose the condition, a doctor will evaluate the types of T and B cells present and their ability to function.


People who have SCID with ADA deficiency have been treated somewhat successfully with enzyme replacement therapy called PEG-ADA. Infants with SCID receive intravenous (through the vein) immunoglobulin or subcutaneous (just under the skin) immunoglobulin to replace their missing antibodies. Gene therapy has been tried for SCID and works, but there have been complications. Bone marrow transplantation is currently the most effective form of treatment.

Last Updated July 17, 2012

Last Reviewed October 14, 2010