Symptoms of SCID occur in infancy and include serious or life-threatening infections, especially viral infections, which may result in pneumonia and chronic diarrhea. As a consequence, they have low numbers of T cells and natural killer cells, and their B cells do not function. Boys with this type of SCID have white blood cells that grow and develop abnormally. X-linked SCID, which is caused by mutations in a gene on the X chromosome, primarily affects male infants. The best-known form of autosomal recessive SCID is caused by adenosine deaminase (ADA) deficiency, in which infants lack the ADA enzyme necessary for T-cell survival. Most often, SCID is inherited in an autosomal recessive pattern, in which both copies of a particular gene-one inherited from the mother and one from the father-contain defects. More than a dozen genes have been implicated in SCID, but gene defects are unknown in approximately 15 percent of newborn-screened SCID infants, according to an NIH-funded study. However, development of a newborn screening test has made it possible to detect SCID before symptoms appear, helping ensure that affected infants receive life-saving treatments. More than 80 percent of SCID infants do not have a family history of the condition. The condition is fatal, usually within the first year or two of life, unless infants receive immune-restoring treatments, such as transplants of blood-forming stem cells, gene therapy, or enzyme therapy. Infants with SCID appear healthy at birth but are highly susceptible to severe infections. Severe combined immunodeficiency (SCID) is a group of rare disorders caused by mutations in different genes involved in the development and function of infection-fighting immune cells.

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