Learn More - Severe Combined Immunodeficiency (SCID)
What is Severe Combined Immunodeficiency (SCID)?
Severe combined immunodeficiency (SCID) is a serious medical condition. Children born with SCID lack adequate immune protection against bacteria, viruses, and fungi and are prone to infections that would not normally cause illness in a person with intact and functional immune system.
Children with SCID are usually diagnosed within the first year of life due to the high frequency and severity of infections. Some of the common organisms that cause mild or no illness in children with healthy immune systems can cause severe infections in children with SCID.
Children affected by SCID can also become ill from live viruses present in some vaccines. These vaccines (such as Chickenpox, Measles, Rotavirus, oral polio and BCG, etc.) contain viruses and bacteria that are weakened and don’t harm children with a healthy immune system. In patients with SCID however, these viruses and bacteria may cause severe, life-threatening infections.
Who gets SCID?
SCID can affect either boys or girls of any race or ethnicity. Defects of at least 14 different genes may result in SCID. The most common form (about 45% of cases) occurs in boys and is inherited as an X-linked disease, meaning that females(mothers) may carry the abnormal gene for the disorder but don’t develop symptoms, whereas male children born to them carry the risk of developing the clinical problems associated with the disease.. SCID is estimated to occur in approximately 1 out of every 50,000 to 100,000 births. (1) (Per [http://ghr.nlm.nih.gov/condition=xlinkedseverecombinedimmunodeficiency])
How is SCID Diagnosed?
The frequency and severity of infections are the most helpful clues that a patient may have a problem with their immune system. For patients with symptoms suggestive of SCID, the following tests can be helpful in making a definitive diagnosis:
- Complete Blood Counts (CBC with differential) – which may show low lymphocyte counts. Patients with SCID are susceptible to infections because they are missing one or more types of lymphocytes.
- T cell, B cell, and NK cell counts –T cells are absent or dysfunctional in all forms of SCID. B cells and NK cells may be absent depending on which type of SCID a patient has. (2)
- Immunoglobulin levels (IgG, IgM, IgA, IgE) – Immunoglobulins (antibodies) are made by the lymphocytes so they are usually low in SCID.
- Specific genetic testing - There are at least 14 known genetic causes of SCID, making it possible to identify an underlying genetic defect in about 90% of cases. Mutations in different genes are accompanied by characteristic immune abnormalities that can assist in making the diagnosis.
- A national committee on newborn screening has now recommended that newborn screening be done for SCID. Various states including Massachusetts, Wisconsin, Maryland, New York and California are initiating or currently doing newborn screening for SCID. As this process expands to other states, most children with SCID will be diagnosed very early in life, increasing their chances for successful treatment.
What is the treatment for SCID?
Treatment for SCID may include the following:
- Medications – especially antibiotic, antifungal, and antiviral medications to treat or prevent active infections
- Avoiding exposure to infections, especially ones that may be difficult to eradicate, such as chicken pox.
- Use immunoglobulin supplementation (IVIg)
- Hematopoietic cell transplantation (HCT)-HCT provides patients with a functioning immune system that is capable of protecting them from infections. It can be a cure for SCID, and is highly effective in many patients, with best outcomes when an HLA-matched sibling donor is available, and if done early in life.
- Enzyme replacement therapy – For patients with the type of SCID caused by deficiency of the enzyme Adenosine Deaminase (ADA), enzyme replacement therapy may be used to enable immune cells to recover and may provide long-term benefit.
- Gene therapy - Patients with some types of SCID have also undergone gene therapy to correct the genetic mutation in their immune cells. While this has been very successful in some patients there have been some serious complications and, at this point, gene therapy is still considered an investigational treatment option.
