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Severe Combined Immunodeficiency (SCID)


James Fernandez

, MD, PhD, Cleveland Clinic Lerner College of Medicine at Case Western Reserve University

Last full review/revision Apr 2021| Content last modified Apr 2021

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.

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 Natural Killer Cells One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 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 of SCID

Diagnosis of SCID

  • 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 of SCID

  • 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 Isolation People who need specific types of care may be put in special care units. Intensive care units (ICUs) are for people who are seriously ill. These people include those who have had a sudden, general... read more ). 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.

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.

More Information about SCID

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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Ataxia-telangiectasia is an inherited disorder that causes poor coordination, dilated capillaries, and deficiencies of the immune system. In addition to immune system issues, this condition typically causes abnormalities in the brain. Which of the following is the part of the brain often affected by ataxia-telangiectasia?
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