Severe combined immunodeficiency disease

Both 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: 

1. reticular dysgenesis, the most severe type, in which the hematopoietic stem cell fails to differentiate into lymphocytes and granulocytes; 
2. Swiss-type agammaglobulinemia, in which the hematopoietic stem cell fails to differentiate into lymphocytes alone; and 
3. enzyme deficiency, such as adenosine deaminase (ADA) deficiency, in which the buildup of toxic products in the lymphoid tissue causes damage and subsequent dysfunction.

Incidence
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.

Signs and symptoms
An extreme susceptibility to infection becomes obvious in the infant with SCID in the first months of life. The infant fails to thrive and develops chronic otitis, sepsis, watery diarrhea (associated with Salmonella or Escherichia coli), recurrent pulmonary infections (usually caused by Pseudomonas, cytomegalo-virus, or Pneumocystis carinii), persistent oral candidiasis (sometimes with esophageal erosions), and possibly fatal viral infections (such as chickenpox).
P. carinii pneumonia usually strikes a severely immunodeficient infant in the first 3 to 5 weeks of life. Onset is typically insidious, with gradually worsening cough, low-grade fever, tachypnea, and respiratory distress. A chest X-ray characteristically shows bilateral pulmonary infiltrates.

Diagnosis
Clinical indications point to the diagnosis. Most infants with SCID suffer recurrent overwhelming infections within 1 year of birth. Some are diagnosed after a severe reaction to vaccination.
Defective humoral immunity is difficult to detect before an infant is 5 months old. Before age 5 months, even normal infants have very small amounts of the serum immunoglobulins (Ig) IgM and IgA, and normal IgG levels merely reflect maternal IgG. However, severely diminished or absent T-cell number and function and lymph node biopsy showing absence of lymphocytes can confirm the diagnosis of SCID.

Treatment
Restoring immune response and preventing infection are the first goals of treatment. Histocompatible bone marrow transplant is the only satisfactory treatment available to correct immunodeficiency.
Because bone marrow cells must be matched according to human leukocyte antigen and mixed leukocyte culture, the most common donors are histocompatible siblings. But bone marrow transplant can produce a potentially fatal graft-versus-host (GVH) reaction, so 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. Administration of 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. However, this approach isn’t effective if the infant already has had recurring infections.

Gene therapy is being used for ADA deficiency.