What Is Severe Combined Immunodeficiency?
Severe combined immunodeficiency (SCID) is very rare genetic disorder that causes life-threatening problems with the immune system. It is a type of primary immune deficiency.
About 1 in 58,000 babies are born with SCID in the U.S. each year.
What Happens in Severe Combined Immunodeficiency?
In a developing baby, the immune system starts in the bone marrow. Stem cells there can become any of three different types of blood cells:
- red blood cells
- white blood cells
White blood cells protect the body from infections and foreign invaders. There are different kinds of white blood cells, including lymphocytes (lim-FOE-sites). Lymphocytes come in two main types: B-cells and T-cells. These cells are key to fighting infections:
- T cells identify, attack, and kill "invaders."
- B cells make antibodies that "remember" an infection and are ready in case the body is exposed to it again.
SCID (pronounced "skid") is a "combined" immunodeficiency because it affects both of these infection-fighting white blood cells.
In SCID, the child's body has too few lymphocytes or lymphocytes that don't work properly.
Because the immune system doesn't work as it should, it can be difficult or impossible for it to battle the germs — viruses , bacteria , and fungi — that cause infections.
What Causes Severe Combined Immunodeficiency?
There are different kinds of severe combined immunodeficiency. The most common type is caused by a problem in a gene found on the X chromosome and affects only males. Females can be carriers of the condition, but because they also inherit a normal X chromosome, their immune systems can fight infections. Males, though, have only one X chromosome, so if the gene is abnormal, the disease appears.
Another form of SCID is caused by a deficiency of an enzyme that is needed for lymphocyte development. Other types of SCID are caused by a variety of other genetic problems.
What Are the Signs & Symptoms of Severe Combined Immunodeficiency?
Babies with SCID may appear healthy at birth, but problems can start soon after, such as:
- failure to thrive
- chronic diarrhea
- frequent, often serious respiratory infections
- oral thrush (a type of yeast infection in the mouth)
- other bacterial, viral, or fungal infections that can be serious and hard to treat, such as:
How Is Severe Combined Immunodeficiency Diagnosed?
Doctors use a simple blood test to screen newborns for many conditions that could cause health problems, such as sickle cell disease and cystic fibrosis. The conditions screened for vary by state, but all now offer screening for severe combined immunodeficiency. Newborn screening for SCID makes early diagnosis possible, and prompt treatment leads to better outcomes.
Babies with a newborn screen suggestive of SCID usually are referred to a doctor specializing in immune deficiencies. The doctor will order other blood tests and possibly genetic testing.
Parents who have a child with SCID or a family history of immunodeficiency might want to consider genetic counseling and early blood testing. Early diagnosis can lead to quick treatment and a better outcome. It may also be possible to test a high-risk baby for the disease before birth if the genetic mutation causing SCID in a family is known.
Children without a known family history of the disease or who don't have a newborn screening often are not diagnosed until 6 months of age or older.
How Is Severe Combined Immunodeficiency Treated?
SCID is a pediatric emergency. Without treatment, babies are not likely to survive past their first birthday. The most common treatment is a stem cell transplant (also called a bone marrow transplant). This means the child receives stem cells from a donor. The hope is that these new cells will rebuild the child's immune system.
The most successful stem cell transplants use cells donated by a sibling. Sometimes, a parent's stem cells are a match. If no family members are suitable donors, doctors may use stem cells from an unrelated donor. Some children with SCID may need chemotherapy before their transplant.
A baby with SCID who gets a stem cell transplant in the first few months of life, before getting any infections, is likely to survive.
Gene therapy as a treatment for some types of SCID has shown promising results in clinical trials, but has some risks so is not yet widely used. Research on gene therapy for SCID is ongoing.
Children with SCID usually are cared for by a medical team of several specialists, such as a:
- pediatric immunologist
- bone marrow transplant physician
- pediatric infectious disease expert
Because children with SCID are at risk for life-threatening infections, they're often started on medicines to help prevent infection. Some might need a regular infusion to replace antibodies.
What Else Should I Know?
Besides medicines and therapies to help prevent infections, other precautions are used. Children with SCID:
- are placed in isolation to help prevent the spread of infection
- should not get live vaccines. Introducing a virus or bacteria to a child with SCID, even in weakened vaccine form, can be dangerous. These include the rotavirus vaccine, chickenpox (varicella) vaccine, the measles, mumps, and rubella (MMR) vaccine, oral polio, BCG, and FluMist vaccines. Anyone living in the same household as the baby also should not receive any live vaccines.
- who need blood transfusions must get blood that has been specially treated to help prevent transfusion-related complications
- may be stopped from breastfeeding until breast milk is checked for a virus that may cause infection
How Can I Help My Child?
While your child is being evaluated for SCID, help reduce the risk of infection by:
- limiting visitors
- avoiding public places
- keeping your baby away from anyone with a cold or infection
- making sure that caregivers wash their hands well before touching your baby
Babies with SCID may need many procedures and repeated hospital stays. That can be stressful for any family. But you're not alone. The care team is there for you and your child before, during, and after treatment. Support groups, social workers, and family friends can also lend a helping hand or understanding ear.
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