Severe Combined Immunodeficiency

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Severe combined immunodeficiency (SCID) is a primary immunodeficiency disease characterized by functional defects in both the humoral and cellular immune systems. Most babies present with lymphopenia. Even the suspicion of SCID constitutes a pediatric emergency. Untreated, SCID is universally fatal, usually within the 1st year of life.


  • Estimated to be 1 in 100,000 births but may be underreported because of early infant mortality
  • Highest in regions where consanguineous marriages take place
  • SCID due to Artemis defects occurs in 1 in 2,500 births in the Navajo and Apache populations because of a founder mutation.

Risk Factors


  • Multiple genetic defects (about 20) can cause SCID.
  • Approximately 50% of cases are X-linked (mutation in the IL-2 γ-chain); the others are autosomal recessive mutations.
  • Classified by the lymphocyte subsets affected (e.g., T− B+ NK+, T− B− NK+)


SCID is caused by mutations in genes required for T-cell growth and development, purine salvage pathway function (e.g., adenosine deaminase [ADA]), expression of histocompatibility (HLA) antigens, and DNA repair.

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