What Is Severe Combined Immunodeficiency Disease (SCID)?

Severe combined immunodeficiency disease (SCID) is a rare genetic condition. It weakens the immune system; the body's defense against infections. An estimated 1 out of 58,000 children have SCID. The condition is more common in boys than it is in girls.

Severe combined immunodeficiency disease can be life-threatening. Another name for it is “bubble boy disease,” after a child named David Vetter, who was born with SCID in 1971. He had to live in a plastic, germ-free "bubble" at Texas Children’s Hospital for 12 years to avoid contact with germs. He died in 1984 due to complications from treatment.

What Causes Severe Combined immunodeficiency Disease?

In children with severe combined immunodeficiency disease, genetic changes cause problems with the white blood cells called T cells, B cells, and NK cells (also known as lymphocytes). The immune system relies on these cells to fight infections. When a child has SCID, either there are not enough lymphocytes or they do not work correctly, so the child’s immune system has difficulty fighting off germs such as fungi, viruses, and bacteria.

There are many different gene changes that can cause SCID. Some of these changes affect boys more frequently than girls. If a child has SCID, the risk for their future siblings to also have SCID may be increased.

About 80% of children with severe combined immunodeficiency do not have a family history of the condition. An estimated 15% of children with SCID do not have a known genetic mutation.

What Are the Types of Severe Combined immunodeficiency Disease?

There are three main types of severe combined immunodeficiency disease. Determining which one a child has involves doing a blood test to measure the number of T cells in the blood (also called the “CD3” count).

• Typical or classic SCID: The child’s CD3 count is less than 300 T cells per microliter of blood. This type of SCID requires urgent medical intervention.
• Atypical or leaky SCID: The child’s CD3 count is more than 300 T cells per microliter of blood. This is the less severe form of SCID but still requires medical intervention.
• Variant SCID: This is a type of atypical/leaky SCID that does not have a known genetic cause.

What are the Symptoms of Severe Combined immunodeficiency Disease?

Children with SCID may look healthy immediately after being born. But they have a hard time fighting any type of infection, even a mild one. They are at risk for frequent or recurrent infections, such as viral, bacterial and fungal infections. These infections are often very serious and can be life-threatening.

SCID can cause other problems, such as:

• Pneumonia
• Chronic diarrhea
• Failure to thrive (not gaining weight or growing) or slow growth

How is Severe Combined immunodeficiency Disease Diagnosed?

All states provide a newborn screening to check for different conditions, including severe combined immunodeficiency. The newborn screening involves a heel prick to collect a small sample of the child's blood. If a child tests positive for SCID, a physician may order more blood tests to confirm the diagnosis.

Thanks to newborn screening, most children with SCID are diagnosed early, often before they start showing symptoms or have a severe infection.

What Are Treatments for Severe Combined immunodeficiency Disease?

Without treatment, children with SCID are at high risk of getting infections that could cause serious illness or death.

Treatments focus on making the immune system stronger and may include:

• Hematopoietic stem cell transplantation (transplant of blood-forming stem cells, also called “bone marrow transplant”)
• Gene therapy
• Enzyme replacement therapy

Most children with SCID will need a stem cell transplant, which may provide them with a working immune system and a cure. A stem cell transplant involves transferring stem cells from a healthy person to the person with SCID. Research shows that when the stem cell donor is a sibling, outcomes are better. If a sibling is not an option, parents or unrelated donors may be able to donate their stem cells.

Gene therapy is another option to cure SCID. Gene therapy involves taking the stem cells from the child's bone marrow. Then, a healthy gene for immune cells is inserted into the stem cells in the laboratory. This “corrects” the stem cells, which are then injected back into the child.

What Is the Long-Term Outlook for Severe Combined immunodeficiency Disease?

Children with severe combined immunodeficiency disease, who are diagnosed early in life and receive high-quality care, have the best chance for a good outcome. If a child with SCID receives a stem cell transplant early in life before acquiring a serious infection, their chances of having a good outcome is much higher.