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Severe Combined Immunodeficiency (SCID)

By James Fernandez, MD, PhD, RJ Fasenmyer Center for Clinical Immunology

Severe combined immunodeficiency is a primary immunodeficiency disorder resulting in low levels of antibodies (immunoglobulins) and low or no T cells (lymphocytes).

  • Most infants with severe combined immunodeficiency develop pneumonia, persistent viral infections, thrush, and diarrhea, usually by age 6 months.

  • Doctors diagnose the disorder by measuring the number of B and T cells and immunoglobulin levels in the blood and by doing tests to evaluate how well B and T cells are functioning.

  • People with this disorder are kept in a protected environment to prevent exposure to possible infections.

  • Treatment involves antibiotics to prevent infections and immune globulin, but the only effective treatments are stem cell transplantation and possibly gene therapy.

Severe combined immunodeficiency is a serious, potentially fatal immunodeficiency disorder. It is present at birth and can be caused by mutations in many different genes. All forms are hereditary.

The most common form results from a mutation in a gene on the X (sex) chromosome (called an X-linked disorder) and occurs almost exclusively in boys.

Another form of the disorder is due to a deficiency of the enzyme adenosine deaminase. This enzyme breaks down a toxic substance in white blood cells, If there is not enough adenosine deaminase, the toxic substance builds up, killing the white blood cells. Thus, there are fewer white blood cells to fight infection.

Because there are no T cells and because B cells cannot produce antibodies (immunoglobulins) without the help of T cells, immunoglobulin levels are low.

Also, natural killer cells do not function normally. Natural killer cells are a type of white blood cell that recognizes and kills abnormal cells (such as certain infected cells and cancer cells).

In people with severe combined immunodeficiency, the immune system provides virtually no protection from bacteria, viruses, and fungi. The result is repeated and persistent infections.

Symptoms

Most infants with severe combined immunodeficiency develop pneumonia, persistent viral infections, thrush, and diarrhea, usually by age 6 months. More serious infections, including Pneumocystis jirovecii pneumonia, can also develop. As a result, infants do not grow and develop normally (called failure to thrive). They may have peeling rashes.

All infants with this disorder have a severely underdeveloped thymus gland.

If not treated, these children usually die before age 1 year.

Diagnosis

  • Blood tests

  • Possibly a screening test for newborns

Symptoms suggest the disorder. Blood tests are done to measure the number of B and T cells and immunoglobulin levels and to evaluate how well B and T cells are functioning.

Some experts recommend screening all newborns for severe combined immunodeficiency with a blood test that determines whether they have abnormal or too few T cells—called the T-cell receptor excision circle (TREC) test. TREC testing of all newborns is now required in many U.S. states. Identifying infants with this disorder early can help prevent their death at a young age.

Doctors may use genetic tests to identify the specific mutation causing the disorder and thus help determine how severe the disorder is and what the prognosis is.

Treatment

  • Reverse isolation

  • Antibiotics, antifungal drugs, and immune globulin to prevent infections

  • Stem cell transplantation

  • Sometimes replacement of adenosine deaminase or gene therapy

People with this disorder are kept in a protected environment to prevent exposure to possible infections (called reverse isolation). In the past, children with this disorder were kept in strict isolation, sometimes in a plastic tent, leading to the disorder being called bubble boy syndrome.

Treatment with antibiotics and immune globulin (antibodies obtained from the blood of people with a normal immune system) helps prevent infections but does not cure the disorder.

The only effective treatment is transplantation of stem cells (for example, from an unaffected sibling with the same tissue type). If transplantation is done by age 3 months, 96% of infants survive.

If adenosine deaminase is deficient and transplantation is not possible, replacement of that enzyme, given by injection, can be partially effective.

Gene therapy may be effective, depending on which form of severe combined immunodeficiency is present. Gene therapy consists of removing some white blood cells from the infant’s bone marrow, inserting a normal gene into the cells, and returning the cells to the infant. However, this therapy cannot be used to treat the X-linked form of severe combined immunodeficiency because leukemia is a risk after such treatments.

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