Severe combined immunodeficiency disease
LIFE-THREATENING DISORDERBoth cell-mediated (T-cell) and humoral (B-cell) immunity are deficient or absent in severe combined immunodeficiency disease (SCID). This results in susceptibility to infection from all classes of microorganisms during infancy.
At least three types of SCID exist: reticular dysgenesis, the most severe type, in which the hematopoietic stem cell fails to differentiate into lymphocytes and granulocytes; Swiss-type agammaglobulinemia, in which the hematopoietic stem cell fails to differentiate into lymphocytes alone; and enzyme deficiency, such as adenosine deaminase (ADA) deficiency, in which
the buildup of toxic products in the lymphoid tissue causes damage and subsequent dysfunction.
the buildup of toxic products in the lymphoid tissue causes damage and subsequent dysfunction.
SCID affects more males than females; its estimated incidence is 1 in every 100,000 to 500,000 births. Most untreated patients die from infection within 1 year of birth.
Causes
SCID is usually transmitted as an autosomal recessive trait, although it may be X-linked. In most cases, the genetic defect seems associated with failure of the stem cell to differentiate into T and B lymphocytes.
Many molecular defects, such as mutation of the kinase ZAP-70, can cause SCID. X-linked SCID results from a mutation of a subunit of the interleukin-2 (IL-2), IL-4, and IL-7 receptors. Less commonly, it results from an enzyme deficiency.
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