SCID
What it means
Severe combined immunodeficiency — profound T-cell defect (± B/NK). X-linked (γc chain) or autosomal recessive (ADA deficiency). Failure to thrive, persistent/severe infections, chronic candidiasis and diarrhoea from the first months. Treat: protective isolation, IVIG, prophylaxis, haematopoietic stem-cell transplant (curative). UK newborn screening (TREC) being rolled out.
Worked example
A young infant with recurrent severe infections, oral thrush and failure to thrive, plus an absent thymic shadow and very low lymphocytes — think SCID; avoid live vaccines.
In the exam
From the first months of life an infant has failure to thrive, persistent candidiasis and repeated severe infections with very low lymphocyte counts.
What settles it
SCID = combined T (± B) defect from early infancy; Bruton's = pure B-cell (no antibody) defect presenting after ~6 months as maternal IgG wanes.
Classically confused with
Bruton's agammaglobulinaemia
Source: StatPearls