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 cells B 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
and T cells T 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 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.
(See also Overview of Immunodeficiency Disorders Overview of Immunodeficiency Disorders Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more .)
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 X-Linked Inheritance Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more ) and occurs almost exclusively in boys. Another form is usually inherited as an autosomal (not sex-linked) recessive Inheritance of Single-Gene Disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more disorder. That is, two genes for the disorder, one from each parent, are required.
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 Antibodies 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 (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
Most infants with severe combined immunodeficiency develop pneumonia, persistent viral infections, thrush Candidiasis Candidiasis is a fungal infection caused by several species of the yeast Candida, especially Candida albicans. The most common type of candidiasis is a superficial infection of... read more 
, and diarrhea, usually by age 6 months. More serious infections, including Pneumocystis jirovecii pneumonia Pneumonia in Immunocompromised People Pneumonia is infection of the lungs. Pneumonia in people whose immune system is weakened or impaired (for example, by acquired immunodeficiency syndrome [AIDS], cancer, organ transplantation... read more , can also develop. As a result, infants do not grow and develop normally (called failure to thrive Failure to Thrive in Children Failure to thrive is a delay in weight gain and physical growth that can lead to delays in development and maturation. Medical disorders and a lack of proper nutrition are causes of failure... read more ). 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 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 ). 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 Stem Cell Transplantation Stem cell transplantation is the removal of stem cells (undifferentiated cells) from a healthy person and their injection into someone who has a serious blood disorder. (See also Overview of... read more (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.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Immune Deficiency Foundation: IDF and SCID center: Comprehensive information on severe combined immunodeficiency, including information on diagnosis and treatment and advice for people affected
