What is X-Linked Severe Combined Immunodeficiency?
X-linked severe combined immunodeficiency (X-SCID) is a disorder of the immune system that causes recurrent, severe infections, fevers, and skin rashes. The condition almost exclusively affects males. Patients with X-SCID are missing two important immune system components, T lymphocytes and natural killer lymphocytes. These patients also have B lymphocytes that do no work. Since the immune system is unable to function properly, X-SCID patients are unable to fight off infections.
Symptoms of X-SCID typically start between three and six months of age. Most male infants with untreated X-SCID will show slower than average growth, develop significant diaper and oral rashes, and will have severe, persistent infections despite active treatment. They may also have absent tonsils and lymph nodes. Babies with an atypical form of X-SCID may have immune system dysfunction, rashes, gastrointestinal problems, and short stature.
X-SCID is inherited as an X-linked condition. This means that typically women are unaffected carriers while males that inherit this condition are affected.
How common is X-Linked Severe Combined Immunodeficiency?
The exact incidence of X-SCID is unknown. It is thought to be the most common form of severe combined immunodeficiency, accounting for up to 50% of cases, and has an estimated incidence of approximately 1 in 115,000 to 1 in 300,000 in the US (diverse population). However, the incidence may vary by region. More information about the true incidence of the condition may come as other countries adopt newborn screening and registries become more established.
How is X-Linked Severe Combined Immunodeficiency treated?
Bone marrow transplantation is the most common form of treatment for X-SCID. Replacement of the bone marrow in a person with X-SCID with the bone marrow of a healthy individual allows the body to generate new, functional blood cells and lymphocytes. Bone marrow transplantation has a significantly higher success rate if performed shortly after birth. Cord blood transplantation may also be an affective treatment of X-SCID. Gene therapy may also be considered for patients that are not good candidates for bone marrow or cord blood transplantation.
What is the prognosis for a person with X-Linked Severe Combined Immunodeficiency?
X-linked severe combined immunodeficiency is almost universally fatal unless a successful bone marrow transplantation or gene therapy is completed. Approximately 90% of infants can be successfully treated with a bone marrow transplant. For infants in which a bone marrow transplant is not completely successful, or for those that are not candidates for a bone marrow transplant, administration of proteins to help the immune system function may be beneficial. For infants in whom bone marrow transplantation, gene therapy, or immunoglobulin infusion is unsuccessful, the average life expectancy is approximately 1-2 years.