Severe combined immunodeficiency (SCID) is a serious medical condition. Children with SCID lack virtually all immune protection against bacteria, viruses, and fungi and are prone to repeated and persistent infections that would not normally cause illness in a person with a normal immune system. In someone with SCID, these infections can be very serious or life-threatening. SCID is estimated to occur in approximately 1 out of every 50,000 to 100,000 births.1 It can affect either boys or girls but the most common type occurs only in males (X-linked). If not treated in a way that restores immune function, children with SCID usually live only a year or two.
Children with SCID usually receive medical attention at a young age (often within the first year of life) because of infections. It is normal for young children to have colds, but a baby with SCID gets infections that are more serious and life threatening, like pneumonia or hepatitis. Some of the most common organisms that cause infections in SCID children may cause only a mild illness or no illness at all in normal kids. Some of these include:
- Pneumocystis jiroveci, which is carried in the airway of most healthy people, can cause a fatal pneumonia (PCP) if not diagnosed and treated appropriately
- Cytomegalovirus (CMV), which is carried by many people in the general population, can cause a fatal pneumonia or hepatitis
- Adenovirus, which causes sore throat and colds in people without SCID, can cause a fatal hepatitis
- Fungal (yeast) infections like thrush (a Candida fungal infection of the mouth) may become severe and difficult to treat in children with SCID.
- Children with SCID can also become ill from live viruses present in some vaccines. Vaccines for chickenpox, measles, rotavirus, etc., which contain viruses that have been crippled in their ability to grow, don’t harm children with a healthy immune system. In patients with SCID however, these viruses may cause severe, life-threatening infections.
Patients with SCID may also have other symptoms that can provide clues to the diagnosis including:
- Persistent diarrhea - common in SCID and may lead to weight loss, malnutrition, and poor growth. May or may not be related to infection.
- Rash - may at times be severe, red, and peeling.
- Autoimmunity – may cause destruction of the patient’s red blood cells (anemia) or platelets (thrombocytopenia).
The most helpful clues that a patient may have a problem with their immune system are infections and other symptoms. In patients that have symptoms suggestive of SCID, the following tests can be helpful in making a definitive diagnosis:
Complete Blood Counts (CBC with differential) – usually shows low lymphocyte counts. Lymphocytes are a group of white blood cells that makes up a significant part of the immune system. 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, B, and NK cells are different types of lymphocytes that play various roles in the immune response. 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 These tests can be coordinated through the
Immunoglobulin levels (IgG, IgM, IgA, IgE) – Immunoglobulins (antibodies) are made by the lymphocytes so they are usually low in SCID. These may be artificially normal during the first three to four months of life because of antibodies that cross the placenta from the mother during pregnancy. If low, these can be replaced in the patient by giving Intravenous Immune Globulin (IVIG).
Specific genetic testing - There are currently 21 known genetic causes of SCID making it possible to identify an underlying genetic defect in about 90 percent of cases. Mutations in different genes are accompanied by characteristic immune abnormalities that can assist in making the diagnosis.