A child born with severe combined immunodeficiency does not have an immune system that works properly. The immune system is made up of cells in the body that protect a person from sickness. A child with SCID doesn’t have that protection.
Though a child with SCID might appear healthy at birth, the child is extremely vulnerable to germs. If a child is exposed to viruses, bacteria, or fungi, he or she will likely grow ill while attempting to fight the infection without a complete immune system. Infections are life-threatening for children with SCID.
SCID is caused by mistakes in the genes. Genes are found in cells and serve as the instructions to make all the different parts of our body. For example, there are lots of genes involved in making a heart or a brain. Numerous genes are needed for a healthy immune system.
Part of the immune system is made up of T cells, B cells, and natural killer (NK) cells, which are all white blood cells. Each type of cell helps fight infection in a special way. But, without T cells, we could not survive. T cells are absolutely necessary for the immune system to fight infection. A dangerously low or absent number of T cells is the cause of SCID.
Several different genes are needed to make a T cell. A defect in any one of these genes can cause SCID. We sometimes name the type of SCID based on the gene that is damaged. For example, in ADA SCID the damaged gene is the ADA, or adenosine deaminase gene.
Sometimes, we categorize SCID based on the white blood count, specifically the number of T, B, and NK cells. For example, SCID with low numbers of T cells and B cells but normal numbers of NK cells is written or described as T-B-NK+ (or T cell negative, B cell negative, NK positive) SCID.
Regardless of which gene is damaged, or if NK cells or B cells are present or absent, all forms of SCID have extremely low or no normal T cells. The newborn screening test that is used to identify babies with SCID detects low or absent T cells. That is why this test can pick up all types of SCID, regardless of the gene that is damaged.
SCID is rare. About 1 out of 58,000 children each year are diagnosed with SCID. That’s about 76 cases of SCID diagnosed annually in the United States.
SCID can affect any infant, regardless of race, gender, ethnicity, or socioeconomic status. It is, however, more common in certain communities, such as the Navajo Nation and the Amish and Mennonite communities.
Symptoms of SCID include:
- Failure to gain weight and grow at a healthy rate
- Life-threatening infections that don’t get better with medicine, such as brain, lung, and blood infections
- Recurrent respiratory tract infections (for example, ear, sinus, or lung infections)
- Skin rashes and skin infections
- Unusually severe yeast infections in the mouth and the diaper area
- Chronic diarrhea
- Infections by germs that usually don't cause infections in healthy people
- Infections in unusual parts of the body such as liver infections
SCID is a life-threatening disease and must be diagnosed and treated promptly. If not treated, a child with SCID will not survive after the first year or two of life.