Severe Combined Immunodeficiency (SCID)

What are the symptoms of severe combined immunodeficiency?

Severe combined immunodeficiency (SCID) can be difficult to detect because babies with the disorder may not show any signs of illness until they develop a telltale infection. In most cases, infections are the first indication that a child may have SCID, though in some children the first sign is afailure to thrive.

What causes SCID?

There are more than 15 recognized kinds of SCID, but the most common type, known as SCID-X1 (for “X-linked severe combined immunodeficiency”), involves a defect in a gene on the X chromosome. Because girls have two X chromosomes while boys have only one, SCID-X1 affects only male children. However, girls can be “carriers” and can pass the disorder on to their own sons later in life.

Different genetic defects cause each form of SCID. But all types of SCID are genetic — meaning they are caused by an error or mutation in the child’s genes.

Even though SCID is a genetic condition, having one child with SCID does not necessarily mean other family members, including siblings, are going to develop the disease. However, it is a good idea to ask your doctor about genetic counseling for you and your other children.

How is severe combined immunodeficiency diagnosed?

A diagnosis of severe combined immunodeficiency(SCID) is usually based on a complete medical history and physical examination of your child. In addition, multiple blood tests — including a complete blood cell count — may be ordered to help confirm the diagnosis.

Newborn screening also plays an important role in detecting SCID before symptoms emerge. A growing number of states test for SCID as part of a mandated newborn screening process. Having a screening performed is especially important, because many infants with SCID show no signs until they actually develop an infection.

Screening does have its limitations. A positive result for SCID on a newborn screen is not the same as a conclusive diagnosis, and screening does not catch all children with SCID and SCID variants. However, it is an important means by which children who may have SCID can be detected early.

What are the treatment options for SCID?

Guarding against infection

Nothing is more important to the health of a child with SCID than warding off potential infections. Your child's clinician can advise you about the exact steps you should be taking to help reduce the risk of infection.

Mothers of newborn children with SCID should discuss the pros and cons of breastfeeding with their clinicians, as some infections can be passed through breast milk.

As a rule, children with SCID should not receive the standard childhood vaccinations. Because the B cells of children with SCID do not function properly, their bodies can't produce the normal antibodies that fight off viruses. Since many vaccines are actually live viruses, they pose too high a risk of infection to be safe for a child with a drastically weakened immune system.

避免潜在感染的其他方法还包括基本预防措施，例如：

• keep your child away from crowds, dirty places or anyone who is ill or seems to be "coming down with something"
• follow a strict hand washing regimen for your child, your family and any visitors
• 在孩子的医生的建议下使用保护性口罩
• give your child antibiotics, antifungal or antiviral medications at the direction of your child's doctor

Antibody infusions

Because your child's body does not have healthy B cells that produce antibodies against infection, he or she may need regular intravenous (IV, administration through a vein) or subcutaneous infusions (administration under the skin) of the antibody immunoglobin (also known as immune globin, gammaglobin, IVIG or SCIG).

Stem cell/bone marrow transplants

Nearly every child with SCID is treated with astem cell transplant, also known as a bone marrow transplant. This is the only available treatment option that has a chance of providing a permanent cure. The bone marrow cells or stem cells are administered through an IV, similar to a blood transfusion.

Stem cells are a versatile type of cell found in bone marrow. These cells have a unique and powerful ability: they can develop into several different types of specialized cells.

对于患有SCID的儿童，将移植的干细胞注射到血液中。然后，它们将成为健康的白细胞，以补充免疫功能 - 基本上为儿童建立了全新的功能性免疫系统。如果免疫系统恢复完整的功能，则可以永久治愈儿童。

The effectiveness of a stem cell transplant for SCID depends on:

• 手术时您孩子的整体健康状况
• the match between your child and donor's bone marrow (If the donated marrow comes from a matched sibling, the procedure has a better than 90 percent success rate.)
• your child's age at the time of the transplant (For optimal success, a baby should have the transplant performed no later than the age of 3 months; some studies have shown that, if stem cell transplants are performed within the first 28 days of a baby's life, the success rate is as high as 95 percent.)

成功的干细胞移植可能存在一些障碍。例如，可能无法提供合适的捐助者。此外，任何移植都有一种被称为移植物抗宿主病的现象的风险。这意味着捐赠的骨髓会攻击接受者，这可能是致命的并发症。

SCID儿童的长期前景是什么？

If your child's immune system is restored to good working order, he or she should be able to enjoy a rich, active adult life, including having a family.

It is important to note, some forms of SCID can run in families. For example, SCID-X1 is passed on to boys from mothers who carry the gene mutation on the X chromosome. Genetic counseling is essential for prospective parents with a family history of SCID or any other immune deficiency.