Severe combined immunodeficiency (SCID)

Severe combined immunodeficiency (SCID) is a rare, often hereditary, disease most often caused by the mutation of an essential gene for the development and functioning of the immune system. The immune system is the body’s natural defence mechanism against infections. Patients with SCID are deficient in essential immune cells. Therefore, they are extremely vulnerable to many infections that may be fatal to them.

Infants with SCID present frequent episodes of pneumonia, ear infections, skin infections, sepsis and other serious infections in the first weeks or months of life. Diarrhea and growth delays generally appear after the infections, which are life threatening. Without a neonatal screening program, children are generally diagnosed only after these serious infections occur.

Defects in different genes may be responsible for SCID. Currently, with the laboratory analyses available, one genetic defect is found in about 90% of cases. The mode of transmission may be autosomal recessive, which means that the affected child has received a copy of the gene that carries the disorder from each of his or her parents, or it may be associated with the X chromosome (the mothers are carriers and the boys get sick if they receive a defective X chromosome). In all cases, the parents may be carriers but not sick.

To diagnose SCID in a young child, a blood sample is taken to see if the T lymphocytes are developing normally. T lymphocytes are part of the white blood cells that fight infections. Without screening, the child would have repeated or serious infections before being diagnosed, which would delay treatment. With neonatal screening, the SCID diagnosis can be made in the first weeks of life. Therefore, newborns with the disorder would benefit from early treatment, which increases their chances of survival and their quality of life.

Since they are highly vulnerable to even the most benign infections, children who have been diagnosed with SCID must be placed in isolation to avoid contracting any illnesses.

Short-term antibiotic and antifungal treatments are administered to prevent infections. The child also receives antibody (immunoglobulin) preparations. Sometimes an enzyme treatment may also be administered to help the immune system.

Since immune cells are produced in the bone marrow, the preferred treatment of SCID is a bone marrow transplant or, in some cases, gene therapy (the introduction of genes into cells to treat a disease).

These treatments are effective in most cases.