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Severe Combined Immunodeficiency

by James Fernandez, MD, PhD

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

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.

Because there are no T cells, B cells cannot produce immunoglobulins, so immunoglobulin levels are low.

Most infants with severe combined immunodeficiency develop pneumonia, persistent viral infections, thrush, and diarrhea, usually by age 6 months. More serious infections, including Pneumocystis pneumonia, can also develop. As a result, infants do not grow and develop normally. They may have peeling rashes, and affected infants have a severely underdeveloped thymus gland.

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

Diagnosis

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 circles (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

People with this disorder are kept in a protected environment to prevent exposure to possible infections. 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 bone marrow 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, in the X-linked form of severe combined immunodeficiency, leukemia is a risk after such treatments.

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