X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency (SCID) is an inherited disorder of the immune system that occurs almost exclusively in males. Boys with X-linked SCID are prone to recurrent and persistent infections caused by certain bacteria, viruses, and fungi. These infections can be very serious or life-threatening. The organisms that cause infection in people with X-linked SCID are described as opportunistic because they ordinarily do not cause illness in healthy people. more...

Many infants with X-linked SCID experience chronic diarrhea and skin rashes, and grow more slowly than other children. Without treatment, affected males usually do not live beyond infancy.

Epidemiology

X-linked SCID is the most common form of severe combined immunodeficiency. The exact incidence is unknown, but the condition probably affects at least 1 in 50,000 to 100,000 births.

Genetics

Mutations in the IL2RG gene cause X-linked severe combined immunodeficiency. The IL2RG gene provides instructions for making a protein that is essential to immune system function. This protein is necessary for the growth and maturation of developing immune system cells called lymphocytes. Lymphocytes defend the body against potentially harmful invaders, make antibodies, and help regulate the entire immune system. Mutations in the IL2RG gene prevent these cells from developing and functioning normally. Without functional lymphocytes, the body is unable to fight off infections.

This condition is inherited in an X-linked recessive pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the disorder; this situation occurs only rarely. Therefore, males are affected by X-linked recessive disorders much more frequently than females.

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