Paeds · allergy-and-immunology
T-cell and combined immunodeficiencies
Also known as Severe combined immunodeficiency · SCID · Combined immunodeficiency · Syndromic immunodeficiency · DiGeorge syndrome immune deficiency · Wiskott-Aldrich syndrome
Fellowship topic on T-cell and combined immunodeficiencies: severe combined immunodeficiency (SCID) as the most severe primary immunodeficiency — a genetic block in T-cell development that is fatal within the first one to two years without curative therapy — and the wider combined and syndromic immunodeficiencies (DiGeorge/22q11.2 deletion, Wiskott-Aldrich, ataxia-telangiectasia, Hyper-IgM/CD40L deficiency, Omenn syndrome, MHC class II deficiency). Why loss of T cells collapses the whole adaptive immune system including antibody production even when B-cell numbers are preserved. SCID classification by immunophenotype (T-B-NK-, T-B-NK+, T-B+NK-, T-B+NK+) and the leading genetic causes (X-linked IL2RG, ADA, RAG1/2, JAK3). The presentation of the SCID infant (severe persistent opportunistic infection — Pneumocystis, CMV, candidiasis, BCGosis — failure to thrive, chronic diarrhoea). Newborn screening by T-cell receptor excision circles (TREC) and confirmation by flow cytometry. The management — protect the infant (no live vaccines, reverse isolation, irradiated blood products, prophylaxis, immunoglobulin replacement), refer urgently to a transplant centre, and cure with haematopoietic stem cell transplantation (best if before 3.5 months and before infection) or gene therapy. The danger of live vaccines, the contraindication to non-irradiated blood, and the role of family screening and genetic counselling.
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Red flags
- A lymphopenic infant with severe, persistent, or opportunistic infection — Pneumocystis, CMV, refractory candidiasis, or chronic diarrhoea — has SCID until proven otherwise; check the absolute lymphocyte count and do flow cytometry today
- Disseminated BCG (BCGosis) after the neonatal vaccine, or vaccine-derived rotavirus disease, is a sentinel presentation of SCID; stop all live vaccines and refer urgently
- Never give non-irradiated blood products to an infant with suspected or confirmed SCID — transfusion-associated graft-versus-host disease from viable donor T lymphocytes is fatal
- An erythrodermic, eosinophilic infant with lymphadenopathy and high IgE has Omenn syndrome — a leaky SCID that is just as much an emergency as classical SCID
- Delay in diagnosis is the chief avoidable cause of death — every day of untreated SCID risks irreversible infection that worsens transplant outcome
Life stages
Care settings
Clinical exam formats
Board mappings
- General Paediatrics and Immunology: recognise the SCID infant — severe persistent opportunistic infection, failure to thrive, lymphopenia — and know that newborn screening (TREC) enables pre-symptomatic cure
- Emergency and resuscitation: stop live vaccines, give irradiated blood products, start prophylaxis and immunoglobulin, and refer urgently to transplant
- Renewed curriculum — Allergy & immunology (LG): SCID phenotypes and genetics, syndromic CID (DiGeorge/22q11.2, Wiskott-Aldrich, ataxia-telangiectasia, Omenn), pathophysiology of T-cell loss and newborn screening by TREC
- Acute and critical care: transplant timing and outcomes, gene therapy, conditioning regimens, and the management of BCGosis and transfusion-associated GvHD
- General Paediatrics / ED: suspect SCID from lymphopenia and opportunistic infection; never give live vaccines; protect with irradiated blood, prophylaxis and immunoglobulin
- Resuscitation: ABCDE, treat active infection, reverse isolation, urgent immunology and transplant referral
- Long Case / Structured discussion: the infant referred after a low TREC newborn screen — confirmation by flow cytometry, protection, the transplant decision, family screening and genetic counselling
- Communication station: explaining SCID, the urgency of transplant, the danger of live vaccines, and the genetic implications to a family
- Level 2 / 3 — Immunology: SCID and combined immunodeficiency as the paediatric emergency; the role of newborn screening and curative HSCT
- Patient safety and quality: avoidable harm in SCID — live vaccines, non-irradiated blood, and delayed diagnosis
- Foundation of Practice (FOP): development of the immune system, the presentation of primary immunodeficiency, and the warning signs
- Applied Knowledge in Practice (AKP): SCID phenotypes, the TREC newborn screen, opportunistic infection, live-vaccine danger, and transplant
- History-taking and management: the infant with recurrent severe infection or a low TREC screen — diagnosis, protection, and curative therapy
- Communication: explaining SCID, transplant, and genetic counselling to a family
- General Pediatrics Content Outline — immunodeficiency: recognise SCID and the syndromic CID; newborn screening and urgent transplant referral
- Allergy & Immunology: SCID phenotypes and genetics, TREC screening, the pathophysiology of T-cell loss, and HSCT outcomes
- Emergency Medicine: the ill lymphopenic infant — stop live vaccines, irradiated blood, prophylaxis, and emergent referral
- Patient Care: recognise and stabilise the SCID infant, protect against iatrogenic harm, and refer for curative HSCT
- Systems-Based Practice: newborn screening programs, equitable access to transplant, and family genetic counselling
- Medical Knowledge: T-cell development, SCID pathophysiology and phenotypes, and the basis of curative therapy
- Medical Expert: SCID diagnosis by flow cytometry and genetic testing, protection of the infant, and the transplant decision
- Health Advocate: universal TREC newborn screening, equitable transplant access, and prevention of live-vaccine harm
- Communicator: explaining SCID, the urgency of curative therapy, and genetic counselling to a family
The same gene, two faces
Classical SCID
Leaky SCID — Omenn syndrome
Severe Combined Immunodeficiency is a Developmental emergency — check the lymphocyte count, do flow cytometry, stop live vaccines, give irradiated blood, and refer for transplant before infection arrives. The rule that saves lives: a lymphopenic infant with severe, persistent, or opportunistic infection has SCID until proven otherwise, and transplant before 3.5 months turns a near-uniformly fatal disease into one with over 90 percent survival. [1] [2]
References12ShowHide
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- [2]Pai SY; Logan BR; Griffith LM; Buckley RH; Parrott RE; Dvorak CC; Kapoor N; Hanson IC Transplantation outcomes for severe combined immunodeficiency, 2000-2009. N Engl J Med, 2014.PMID 25075835
- [3]McDonald-McGinn DM; Sullivan KE; Marino B; Philip N; Swillen A; Vorstman JA; Zackai EH; Emanuel BS 22q11.2 deletion syndrome. Nat Rev Dis Primers, 2015.PMID 27189754
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- [7]Notarangelo LD Genetically-determined defects of T cell development. Allergy Asthma Proc, 2024.PMID 39294907
- [8]Óskarsdóttir S; Boot E; Crowley TB; Loo JCY; Arganbright JM; Armando M Updated clinical practice recommendations for managing children with 22q11.2 deletion syndrome. Genet Med, 2023.PMID 36729053
- [9]Shearer WT, Fleisher TA, Buckley RH, et al. Recommendations for live viral and bacterial vaccines in immunodeficient patients and their close contacts. J Allergy Clin Immunol, 2014.PMID 24582311
- [10]Lankester AC; Neven B; Mahlaoui N; von Asmuth EGJ; Courteille V; Alligon M Hematopoietic cell transplantation in severe combined immunodeficiency: The SCETIDE 2006-2014 European cohort. J Allergy Clin Immunol, 2022.PMID 34718043
- [11]Kreins AY; Dhalla F; Flinn AM; Howley E; Ekwall O; Villa A European Society for Immunodeficiencies guidelines for the management of patients with congenital athymia. J Allergy Clin Immunol, 2024.PMID 39303894
- [12]van Oers NSC; Sullivan KE The systemic effects of 22q11.2 deletion syndrome on immunity. J Hum Immun, 2026.PMID 41608124