Severe Combined Immunodeficiency (SCID) Symptoms & Causes

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Severe combined immunodeficiency (SCID) is very rare, affecting between 50 and 100 children born in the U.S. every year. Because it is a genetic disorder (caused by an error in the genes), in affected children, the condition is present at birth.

Essentially, children with SCID lack the ability to produce an immune system. The human immune system constantly patrols, protects and defends the body from all types of “enemies,” including:

  • bacteria
  • viruses
  • parasites
  • fungi

The immune system first takes root in a developing fetus’ bone marrow. That’s where some stem cells eventually mature into the two cell types that play the biggest role in warding off infection: T cells (white blood cells that identify and attack perceived “invaders”) and B cells (white blood cells that produce antibodies against infection).

A child with severe combined immunodeficiency (SCID):

  • cannot produce T cells
  • does not have working B cells (because there are no T cells for them to work with)
  • has virtually no immune system

Without a functioning immune system, a child with SCID has no way of warding off infections. From the time she is born, she will be at constant risk for:

  • pneumonia
  • chronic diarrhea
  • thrush infections of the mouth and skin
  • many other types of infection, including severe viral infections and invasive bacterial or fungal infections

While a child's specific course of treatment depends on her individual symptoms and circumstances, nearly every child with SCID undergoes stem cell transplant (also known as a bone marrow transplant). Because stem cells have the special ability to develop into other types of cells, a transplant will give the child’s body an essential new source of healthy white blood cells—rebuilding his immune system and allowing him to successfully ward off infections.


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.

Signs and symptoms

What are the symptoms of SCID?
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 a failure to thrive.

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