Severe Combined Immunodeficiency

What Is SCID?

SCID is actually a group of inherited disorders that cause severe abnormalities of the immune system. These disorders lead to reduced or malfunctioning T- and B-lymphocytes, the specialized white blood cells made in the bone marrow to fight infection. When the immune system doesn't function properly, it can be difficult or impossible for it to battle viruses, bacteria, and fungi that cause infections.

SCID is called "combined" immunodeficiency because it affects the function of two kinds of infection-fighting cells where other immune system diseases involve only one. There are several forms of SCID. The most common type is caused by a problem in a gene found on the X chromosome and affects only males. Females may be carriers of the condition, but because they also inherit a normal X chromosome, their immune systems usually can fight infections normally. Males, on the other hand, only have one X chromosome.

Another form is caused by a deficiency of the enzyme adenosine deaminase (ADA). Other cases of SCID are caused by a variety of other genetic defects.

Diagnosing SCID

Classic signs of SCID include an increased susceptibility to infection and failure to thrive as a result of infections. A baby with SCID may have recurrent bacterial, viral, or fungal infections that are much more serious and less responsive to treatment than would normally be expected. These can include ear infections (acute otitis media), sinus infections (sinusitis), oral thrush (a type of yeast infection in the mouth), skin infections, meningitis, and pneumonia. Infants with SCID may also have chronic diarrhea. If a child has these symptoms, a doctor will test for SCID or other types of immune deficiency.

Parents who have a child with SCID or a family history of immunodeficiency might want to consider genetic counseling and early blood testing, since early diagnosis can lead to prompt treatment and improve the chances of a good 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. Babies born with SCID can have a healthy immune system if they are treated early in life.

Most children without a known family history of the disease are not diagnosed until 6 months of age or older.

Treating SCID

When a child is diagnosed with SCID, a referral typically is made to a doctor who specializes in treating immune deficiencies — usually a pediatric immunologist or pediatric infectious disease expert.

It's important to prevent infections in kids with SCID, so your doctor may prescribe antibiotics to prevent infection and advise keeping the child away from crowds and sick people.

Children with SCID should not be immunized with live viruses — like the chickenpox (varicella) or measles, mumps, and rubella (MMR) vaccines — because they lack the normal defense of antibodies to the viruses. Introducing a virus, even a weakened vaccine virus, can be dangerous.

Doctors may also administer an infusion of intravenous immune globulin (IVIG) to help the body fight infection.

The most effective treatment for SCID is a stem cell transplant. This is when stem cells — cells found primarily in the bone marrow from which all types of blood cells develop — are introduced into the body in the hopes that the new cells will rebuild the immune system.

To provide the best chances for success, a transplant is usually done using the bone marrow of a sibling. However, a parent's marrow might also be acceptable. Some children do not have family members who are suitable donors — in such cases, doctors may use stem cells from an unrelated donor. The likelihood of a good outcome also is higher if the transplant is done early, within the first few months of life, if possible.

Some SCID patients require chemotherapy before their transplant. Chemotherapy will destroy cells in the bone marrow to make room for the donated cells and help prevent the child's immune cells from attacking the donated cells. Other kids with SCID may not need such treatment, especially if they have very few immune cells to start with. The use of pre-transplant chemotherapy depends on the severity of the immune deficiency, the type of SCID, the donor used, and the transplant center.

In cases of SCID caused by a missing enzyme, the enzyme can be replaced via a weekly injection. This is not a cure and these children must receive the injections for the rest of their lives.

Another treatment approach currently being studied is gene therapy. This involves removing cells from a child with SCID and inserting healthy genes into them, then transplanting them back into the child. When they find their way to the bone marrow, they can start to produce healthy immune cells. Gene therapy has been successful for some patients with certain types of SCID, but a few children treated with it developed complications, so it has not yet become routine treatment. New trials of gene therapy are ongoing.

Caring for Your Child

Babies who have had bone marrow transplants may need additional treatment with antibiotics or immunoglobulins. Your doctor will advise you about these.

Until your child's immune system develops adequate protection after a bone marrow transplant, you can help reduce the risk of infection by having your child wear a mask. A mask can also serve as a signal to others that your child is trying to avoid infection.

Understand that infants with SCID may have to endure many painful procedures and repeated hospital stays. And that can be stressful and difficult for the entire family. Luckily, this doesn't have to be handled alone: support groups, social workers, and family friends often can lend a helping hand. It's important to reach out for support during this time.

When to Call the Doctor

If you're concerned that your child has more frequent infections than usual, discuss the possibility of immune deficiency with your doctor. If your child has a serious infection, contact your doctor immediately.

Because early treatment is more successful, you can improve your child's chances of developing a healthy immune system by acting quickly. If your child has SCID, any illness merits close medical attention.

Reviewed by: Steven Dowshen, MD
Date reviewed: November 2009