Severe combined immunodeficiency (scid)

[Sommer]

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 cells 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.

(See also Overview of Immunodeficiency Disorders.)
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 is usually inherited as an autosomal (not sex-linked) recessive 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 (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.