Severe combined immunodeficiency- X-linked


A rare inherited disorder where a genetic mutation results in a deficiency of normal lymphocytes which results in a poorly functioning immune system. Sufferers are prone to infections which would probably be harmless in people with fully functioning immune systems.


* Recurring infections * Persistent infections * Life-threatening infections * Chronic diarrhea * Skin rashes * Slow growth * Infant death if untreated * Low functional lymphocyte level


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 is due to a mutation of a subunit of the interleukin (IL)-2, IL-4, and IL-7 receptors. Less commonly, it results from an enzyme deficiency. 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.


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Treatment aims to restore the immune response and prevent infection. Histocompatible bone marrow transplantation is the only satisfactory treatment available to correct immunodeficiency. Because bone marrow cells must be human leukocyte antigen and mixed leukocyte culture matched, the most common donors are histocompatible siblings. However, because bone marrow transplant can produce a potentially fatal graft-versus-host (GVH) reaction, newer methods of bone marrow transplant that eliminate GVH reaction (such as lectin separation and the use of monoclonal antibodies) are being evaluated. Fetal thymus and liver transplants have achieved limited success. Immune globulin administration may also play a role in treatment. Some SCID infants have received long-term protection by being isolated in a completely sterile environment. However, this approach isn’t effective if the infant already has had recurring infections. Gene therapy is being used to treat ADA deficiency.