Severe combined immune deficiency (SCID)
Severe combined immune deficiency (SCID)
Alternative names
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- “Bubble baby” disease
- Primary immune deficiencies
- Severe T-Cell lymphopaenia
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About the condition
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SCID is the most severe form of primary or inherited immune deficiency. It results in low or absent T
lymphocytes and is due to genetic defects, which cause interruption to the development and
maturation of T lymphocytes. Some forms of SCID can also affect the development of B lymphocytes and
NK lymphocytes. Lymphocytes are important cells of the immune system responsible for fighting
infections. Without them, babies can quickly become overwhelmed by severe bacterial, viral or fungal
infections.
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Inheritance
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The most common form of SCID is X-linked and the remainder are autosomal recessive.
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Year started
screening in WA
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2023
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Incidence in WA
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1:50,000 (estimated in WA)
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Defect
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Multiple genetic defects have been identified. All lead to failure to produce functional T
lymphocytes or increased destruction of T cells due to accumulation of toxic metabolites. Some
conditions may also affect NK and B lymphocyte development.
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Symptoms if untreated
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Infants with SCID appear well at birth but can quickly develop overwhelming infection as they are
unable to clear pathogens. They present with:
- Recurrent and or severe infection
- Opportunistic infections eg. Pneumocystis jirovecii
- Growth faltering
- Chronic gastrointestinal losses
- Some forms of SCID may present with severe early onset eczema
- SCID is usually fatal by 12-18 months if left untreated
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Determinants on bloodspot screening
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Reduced numbers of T-cell Receptor Excision circles (TRECs) which are a bi-product of T cell
development. SCID newborn screening will also measure KRECs – Kappa-deleting Recombination
excision circles which are a bi-product of B cell development.
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Diagnostic tests
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- Immunoglobulins
- Lymphocyte subsets including naïve T cells
- Confirmatory testing via genetic sequencing is used to identify the specific gene defect which
will direct therapy decisions
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Treatment
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Immediate treatment includes antimicrobial therapies to prevent infection (antibiotics, antiviral and
antifungal medications). Definitive treatment usually requires a bone marrow or stem cell transplant
but in some instances the defect can also be treated with gene therapy or thymic transplant (less
common).
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Screening issues
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Preterm babies, unwell babies or babies whose mothers are on immunosuppression medications may have
transiently low T lymphocytes at birth which spontaneously resolve.
There are other more common conditions resulting in low T cell levels. These patients may also be
detected by SCID NBS methods.
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