The most common form of SCID has an X-linked recessive pattern of inheritance, and is
therefore referred to as X-linked SCID. When a gene is located on the X chromosome, males are more often
affected than females. Males do not have a second X chromosome to compensate for the defective one.
They need to inherit just one bad copy of the gene to have the disorder. Females, on the
other hand, have two X chromosomes. If they inherit one defective X chromosome, they still have its
healthy pair. They don't develop the disorder, but they still carry the faulty gene and
can pass it to their children.
What are the symptoms of SCID?
Symptoms usually appear in the first few months of life. Because the immune system cannot
protect the baby's body, babies with the disorder tend to get one infection after another. Some of these
infections may be life-threatening, including pneumonia (lung infection), meningitis (brain infection),
and sepsis (blood infection).
To make matters worse, SCID patients often don't respond to the antibiotics used to treat
bacterial infections. They may suffer more frequently from ear infections, sinus infections, a chronic cough,
and rashes on the skin.
Early diagnosis of SCID is very important, because without quick treatment,
children with the disease aren't likely to live past age 2.
How do doctors diagnose SCID?
SCID can be identified before the baby is born either (1)by removing and testing cells from the placenta
(chorionic villus sampling or CVS), or (2) by removing and testing a sample of the fluid surrounding the baby (amniocentesis).
Most babies are diagnosed with SCID in the first 6 months of life. The most common screening
methods are an immune function test, and a blood test that detects low white blood cell counts, as well as low
levels of immune cells (T cells and B cells).