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Introduction

Severe combined immunodeficiency (SCID) is a severe primary immunodeficiency caused by one of several genetic mutations that impair T-cell development with subsequent impaired production of immunoglobulin G (IgG) and immunoglobulin A (IgA) antibodies.  Patients have both cellular and humoral immune deficiency and are at risk for severe, recurrent viral, fungal, and opportunistic (eg, Pneumocystis jiroveci) infections.

Pathogenesis

SCID is caused by one of several genetic mutations that lead to T-cell lymphopenia.  Some SCID mutations also directly impair B and natural killer (NK) cell development.  However, even if B-cell development is not directly impacted, T-cell signaling is required for production of IgG and IgA antibodies (ie, T-cell–induced immunoglobulin class switching).  In these cases, B cells are present but nonfunctional (ie, cannot produce antibodies).  Therefore, in all forms of SCID,

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Tables

Table 1