Definition:

Severe combined immunodeficiency (SCID) comprises a heterogeneous group of monogenic disorders that result in early-onset severe infections by a range of pathogens (such as bacteria, viruses and fungi). Typically, patients with SCID have a severe defect in T-cell differentiation, along with direct or indirect impairment of B-cell development and function. SCIDs with lack of circulating T cells but a normal number of B cells accounts for 30 to 50% of all cases of human SCIDs. The most frequent form of SCID, X-linked SCID (SCID-X1), is caused by mutations in the gamma-chain-encoding gene. Gamma-chain is a common subunit shared by several cytokine receptors. It results in an absence of both mature T lymphocytes and NK lymphocytes. The second most common variant is autosomal recessive and due to mutations of the JAK3 gene. IL-7R alpha gene mutations result in a pure T-cell deficiency. Rare cases of SCID consisting of pure T-cell deficiencies have been attributed to defects in key proteins involved in pre-TCR/TCR signaling. Deficiency in the CD45 phosphatase has been reported in two cases of SCID, while defects in CD3 delta, CD3 epsilon and CD3 zeta have also been described.