Severe Combined Immunodeficiency
What is Severe Combined Immunodeficiency (SCID)?
SCID is a group of very rare-and potentially fatal-inherited disorders related to the immune system. The immune system normally fights off attacks from dangerous bacteria and viruses. People with SCID have a defect in their immune system that leaves them vulnerable to potentially deadly infections.
There are several types of SCID. The most common form is caused by a mutation in the SCIDX1 gene located on the X chromosome. This gene encodes a protein that is used to construct a receptor called IL2RG (interleukin-2 receptor). These receptors reside in the plasma membrane of immune cells. Their job is to allow two types of immune cells - T cells and B cells - to communicate. When the gene is mutated, the receptors cannot form and are absent from immune cells. As a result, the immune cells can't communicate with one another about invaders in the environment. Not enough T and B cells are produced to fight off the infection, and the body is left defenseless.
Another form of SCID is caused by a mutation on chromosome 20 and is characterized by a deficiency of the enzyme adenosine deaminase (see ADA Deficiency).
How do people get SCID?
The most common form of SCID exhibits an X-linked recessive pattern of inheritance, and is therefore referred to as X-linked SCID. When a gene is located on the X chromosome, males are more often affected than females. Males do not have a second X chromosome to compensate for the defective one. They only need to inherit one bad copy of the gene to have the symptoms of the disorder. Females, on the other hand, have two X chromosomes. If they inherit one defective X chromosome, they still have its healthy pair. They don't develop symptoms of the disorder, but they still carry the faulty gene and can pass it on to their children.
What are the symptoms of SCID?
Symptoms usually appear in the first few months of life. Because the immune system cannot protect the baby's body, babies with the disorder tend to get one infection after another. Some of these bacterial infections may be life-threatening, including pneumonia (lung infection), meningitis (brain infection), and sepsis (blood infection).
To make matters worse, SCID patients often don't respond to the antibiotics used to treat bacterial infections. They may suffer more frequently from ear infections, sinus infections, a chronic cough, and rashes on the skin.
Early diagnosis of SCID is very important, because without quick treatment, children with the disease aren't likely to live past age 2.
How do doctors diagnose SCID?
SCID can be identified before the baby is born by removing and testing cells from the placenta (chorionic villus sampling or CVS), or by removing and testing a sample of the fluid surrounding the baby (amniocentesis).
Most babies are diagnosed with SCID in the first 6 months of life. The most common screening methods are an immune function test, and a blood test that detects low white blood cell counts, as well as low levels of immune cells (T cells and B cells).
How is SCID treated?
Children with SCID must be careful to stay away from germ-rich environments (such as day care centers and crowded shopping malls) where they could pick up a potentially life-threatening infection.
The most effective treatment is a bone marrow transplant. Unspecialized stem cells (that will form blood and immune cells) are taken from the bone marrow of a healthy donor and injected into the SCID patient. Ideally, these new cells will stimulate the production of the needed immune cells. Transplants done within the first few months of life are most successful. The tissue must be "matched" to the patient, however, which can limit the usefulness of this therapy. Siblings make the best donors as their cells likely contain a similar genetic makeup.
Gene therapy for this disorder may soon be possible. This therapy would compensate for the faulty gene by injecting healthy copies of the gene into a patient's bone marrow stem cells.
Interesting facts about SCID
About 1 out of every 100,000 babies is born with SCID.
SCID is sometimes called Bubble Boy disease. In the 1970s, a boy named David Vetter had to live in a plastic bubble for 12 years because of SCID.