Severe Combined Immunodeficiency (page 3)

Right after they're born, babies are partially protected from infections by antibodies transmitted to them by their mothers. (Antibodies are special chemicals made by the body to fight infections.) Within the next few months, though, their immune systems develop and begin to assume responsibility for fighting off infections. But sometimes, babies have immune deficiencies and they don't have the ability to fight off routine infections on their own.

The immune system has many parts. The symptoms of immune deficiency depend on what part of the immune system is affected. Immune deficiencies can range from mild to life-threatening. One example of a life-threatening immune deficiency is severe combined immunodeficiency (SCID). "Combined" means that multiple parts of the immune system are affected.

SCID, an uncommon disease, can be successfully treated if it's identified early; otherwise, it is almost always fatal within the first year of life.

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 and the thymus gland 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. There are 14 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 can fight infections normally. Males, on the other hand, only have one X chromosome, so if the gene is abnormal the disease appears.

Another form of SCID is caused by a deficiency of an enzyme (adenosine deaminase or ADA), which is necessary for lymphocytes to develop. 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 will 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 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 ensure 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. Technology has made it possible to test newborn babies for SCID using the blood spot that is collected at birth. And, many states in the United States have SCID screening for newborns.

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

Treating SCID

SCID is a pediatric emergency. 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 babies with SCID, so doctors may prescribe antibiotics to prevent infection and admit the child to an isolation room in the hospital.

Children with SCID cannot 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 to a child with SCID, even a weakened vaccine virus, can be dangerous. Children with SCID can only receive blood transfusions with blood that has been irradiated to kill white blood cells, because live white cells might attack the body.

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

The curative 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 inhabit and rebuild the immune system of the child with SCID.

To provide the best chances for success, a transplant is usually done using the bone marrow of a sibling. However, a parent's marrow is also 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 greater if the transplant is done early, within the first few three 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.