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

Severe combined immunodeficiency is the most serious immunodeficiency disorder. It can be caused by several different genetic defects. All forms are hereditary. One 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, 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 all affected infants have a severely underdeveloped thymus gland. If not treated, these children usually die before age 1 year.

Diagnosis and Treatment

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.

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 helps but does not protect against severe viral infections. The only effective treatment is transplantation of bone marrow stem cells from an unaffected sibling with the exact same tissue type or from a parent with a half-matched 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 one form of severe combined immunodeficiency, leukemia is a risk after such treatments.

Last full review/revision September 2008 by Rebecca H. Buckley, MD