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Severe combined immunodeficiency (SCID): An overview

Francisco A Bonilla, MD, PhD
Section Editor
E Richard Stiehm, MD
Deputy Editor
Elizabeth TePas, MD, MS


The term "primary immunodeficiency" denotes diseases resulting from inherited defects of the immune system. Many distinct disorders have been described [1]. Combined immunodeficiency syndromes are a heterogeneous group of disorders arising from a disturbance in the development and function of both T and B cells (cellular and humoral immunity). Combined immunodeficiencies are termed "severe" when they lead to early death from overwhelming infection, typically in the first year of life.

An overview of severe combined immunodeficiency (SCID), including clinical manifestations and diagnosis, is presented here. The major combined immunodeficiencies, including multiple causes of SCID, are discussed in detail separately. (See "Severe combined immunodeficiency (SCID): Specific defects" and "X-linked severe combined immunodeficiency (SCID)" and "Adenosine deaminase deficiency: Pathogenesis, clinical manifestations, and diagnosis" and "T-B-NK+ SCID: Pathogenesis and genetics" and "Severe combined immunodeficiency (SCID) with JAK3 deficiency" and "ZAP-70 deficiency" and "Combined immunodeficiencies" and "CD3/T cell receptor complex disorders causing immunodeficiency".)

The humoral immunodeficiencies and disorders involving phagocytic and complement defects are presented separately. (See "Primary humoral immunodeficiencies: An overview" and "Primary disorders of phagocytic function: An overview" and "Inherited disorders of the complement system".)


A study using data from newborn screening for T cell lymphopenia in the United States found an incidence of 1 in 58,000 live births (95% CI, 1 in 46000 to 1 in 80,000) for typical SCID, leaky SCID, and Omenn syndrome [2]. The incidence of autosomal recessive SCID is higher in cultures in which consanguineous marriage is common [3,4]. (See 'SCID classification' below and "Newborn screening for primary immunodeficiencies".)


SCID is a syndrome caused by mutations in different genes whose products are crucial for the development and function of both T and B cells. In some cases, the molecular defect results in only T cell deficiency, while B cells are normal. However, serious T cell dysfunction precludes effective humoral immunity since B cells require signals from T cells to produce antibody.

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Literature review current through: Oct 2017. | This topic last updated: Apr 11, 2017.
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