Severe Combined Immunodeficiency Disease

Cell-mediated (T-cell) and humoral (B-cell) immunity are either deficient or absent in severe combined immunodeficiency disease (SCID), which predisposes the patient to infection from all classes of microorganisms during infancy. Defective B-cell and T-cell function may also cause other immunodeficiency disorders. Three types of SCID have been identified: 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 deficiency, in which the buildup of toxic products in the lymphoid tissue causes damage and subsequent dysfunction.

SCID affects more males than females and occurs in 1 in every 100,000 to 500,000 births. Most untreated infants die of 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 cells. Many molecular defects, such as mutation of the kinase ZAP­70, can cause SCID. X-linked SCID results from a mutation of a subunit of the interleukin (IL)-2, IL-4, and IL-7 receptors. Less commonly, SCID results from enzyme deficiency.

Another theory is that the thymus or bursa equivalent fails to develop normally or that there is a defect in the thymus and bone marrow, which are responsible for T- and B-cell development.       

Symptoms

Symptoms of SCID usually become apparent within the first year. The following are the most common symptoms of SCID. However, each child may experience symptoms differently. Symptoms may include:

• numerous, serious, and/or life-threatening infections that are not easily treated and do not respond to medications (as they would in children without SCID), including the following:
  
  • pneumonia - infection of the lungs.
  • meningitis - infection of the brain.
  • sepsis - infection in the bloodstream.
• other infections, including the following:
  • chronic skin infections
  • yeast infections in the mouth and diaper area
  • diarrhea
  • infection of the liver

Diagnostic tests

Defective humoral immunity is hard to detect before an infant reaches age 5 months. Before that age, ever normal infants have only small amounts of serum IgM and IgA, and normal IgG levels merely reflect maternal IgG. Tests that show a severely diminished or absent T-cell number and function and a lymph node biopsy that shows an absence of lymphocytes can confirm a diagnosis of SCID.

Treatment

The goal of treatment is to restore immune response and prevent infection. Specific antibiotic therapy is used to treat infection. The patient needs histo­compatible bone marrow transplantation to correct immunodeficiency. Bone marrow cells must be matched for histocompatibility, using both human leukocyte antigen and mixed leukocyte culture, so siblings usually serve as donors. Parental marrow can now be used successfully after it's depleted of T cells, which could cause fatal acute graft-versus-host disease. T cells are removed with monoclonal antibodies or lectin columns. Immunoglobulin may be given I.V. with dosages varying according to the severity of the illness.

Fetal thymus and liver transplantation have met with limited success. Immune globulin may also play a role in treatment. Some SCID infants have received long-term protection by being isolated in a completely sterile environment. This approach doesn't work for an infant who already has recurring infections.

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