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Severe combined immunodeficiency (SCID) is a fatal primary immune condition characterized by the absence of both humoral and cellular immunity with severe lymphopenia. Patients with SCID have reduced numbers and function of both T and B lymphocytes (and in some cases, natural killer [NK] cells) and hypogammaglobulinemia. If left untreated, babies with SCID most often die within 1 to 2 years of life because of severe, recurrent infections.

Since SCID was initially described over 50 years ago, 15 specific genetic mutations that contribute to disease development have been identified. The most common type is X-linked SCID (SCID-X1), which accounts for approximately 46% of cases in the United States (Figure 111-1). SCID can also be inherited as an autosomal recessive disease that results from deficiencies in adenosine deaminase (ADA), Janus kinase 3 (JAK3), interleukin 7 receptor α chain (IL7R), recombination activating genes (RAG1 or RAG2), Artemis gene, CD45, and others (Table 111-1).

Table 111-1Classification of Severe Combined Immunodeficiency (SCID)
FIGURE 111-1

The relative frequencies of particular genetic mutations were evaluated in 174 consecutive cases of human severe combined immunodeficiency (SCID) evaluated at Duke University over 30 years. The most common type is X-linked SCID, which disables common gamma (γc) signaling. ADA, adenosine deaminase; JAK3, Janus kinase 3; IL7R, interleukin 7 receptor α chain; RAG1 or RAG2, recombination activating genes 1 or 2. (Reproduced with permission ...

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