In the time following birth, newborns are protected by immunity transmitted to them by their mothers. Within the next few months, though, their immune systems develop and begin to assume responsibility for fighting off infections. But it doesn't take long to determine that a few babies don't have the ability to fight off routine infections on their own.

Severe combined immunodeficiency (SCID) is a rare immune deficiency. There are many other immune deficiencies that may result in recurrent infections, but some children are born with an incomplete, or deficient, immune system. The symptoms of immune deficiency depend on what part of the immune system is affected and can range from mild to life-threatening. SCID is a primary immune deficiency that can be successfully treated if it's identified early. Otherwise, it's often fatal within the first year.

What Is SCID?
SCID is actually a group of inherited disorders characterized by a lack of immune response. It occurs when a child lacks lymphocytes, the specialized white blood cells that the body uses to fight infection.

Lymphocytes are made in the bone marrow. Some lymphocytes move to the thymus gland, where they become T cells. B cells remain in the bone marrow to mature. Each specialized type of cell is responsible for a particular immune response. T cells attack antigens (usually invading germs) directly and help the body reject foreign tissue. T cells are also needed to activate the B cells that produce immunoglobulins (antibodies) to fight specific invaders.

If a child's immune system isn't functioning properly, it can be difficult or impossible to fight off viruses, bacteria, and fungi that cause infections.

Sometimes called "bubble boy disease," SCID became more widely known in the 1970s when the world learned of David Vetter, a boy with SCID who lived for 12 years in a plastic, germ-free bubble.

There are several forms of SCID. The most common type is linked to the X chromosome, which makes it more common among males. Another form is linked to a deficiency of the enzyme adenosine deaminase (ADA). Other cases of SCID are caused by a variety of other genetic defects.

Children with untreated SCID rarely live to age 2.

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