Leaky SCID
A phenotypic variant of severe combined immunodeficiency in which hypomorphic mutations permit partial lymphocyte development, resulting in a less severe but still life-threatening immune defect.
Full Definition
Leaky SCID (also termed atypical SCID or partial SCID) arises when hypomorphic mutations in SCID-causing genes—most commonly RAG1, RAG2, or IL2RG—allow residual gene function sufficient to support a limited degree of lymphocyte maturation. Affected children typically have low but detectable T- and/or B-cell counts with restricted repertoire diversity, distinguishing them from classic SCID presentations with absent lymphocytes. Clinical presentation is heterogeneous and may be delayed beyond the newborn period, complicating screening and diagnosis. Importantly, leaky SCID may be missed on standard newborn screening if T-cell receptor excision circle (TREC) counts are borderline. Editors must use 'leaky' as a technical modifier and not substitute colloquial alternatives; the term is widely accepted in the primary immunodeficiency literature.
Usage
Usage note: 'Leaky SCID' is accepted technical terminology; do not replace with informal paraphrases. Distinguish clearly from classic SCID and from combined immunodeficiency (CID) without the SCID severity threshold.
In Context
- "Newborn screening returned a borderline-low TREC value; subsequent evaluation confirmed leaky SCID due to a hypomorphic RAG2 mutation." — Newborn screening follow-up report
- "The review article used 'partial SCID' and 'leaky SCID' interchangeably; the editor standardised to 'leaky SCID' as the preferred term per the IUIS classification." — Manuscript style harmonisation note