Severe combined immunodeficiency

A group of rare inherited immunodeficiencies characterized by profound T- and B-cell dysfunction, leading to extreme vulnerability to infections in early infancy.

• SCID is the most severe primary immunodeficiency—fatal in infancy without treatment. Early detection (newborn TREC screening) allows timely curative therapy (e.g. bone marrow transplant), transforming an otherwise lethal disease.

• Onset at 3–6 months of age (after maternal IgG wanes) with recurrent severe infections: e.g. persistent thrush, chronic diarrhea, Pneumocystis pneumonia; failure to thrive.
• Opportunistic pathogens (fungal, viral, protozoal) and high susceptibility to live vaccine strains (e.g. rotavirus) due to absent T-cell immunity.
• Physical exam: absence of lymphoid tissue (no tonsils, no lymph nodes); absent thymic shadow on chest X-ray (thymic aplasia).
• Lab findings: lymphopenia (especially very low T cells); B cells may be present but nonfunctional (require T help). Newborn screening shows very low/absent TREC levels (T-cell receptor excision circles).

1. Maternal IgG protects newborns for ~6 months; consider SCID in infants who appear healthy at birth but then develop severe infections after a few months.
2. X-linked SCID (IL2RG gene) is most common (∼50%, only in boys); autosomal recessive forms include ADA deficiency, JAK3 mutation, RAG1/2 mutations, etc., each causing distinct T/B/NK cell absence patterns.
3. Differentiate from other causes of T-cell lymphopenia: e.g. DiGeorge syndrome (thymic aplasia) also lacks T cells but presents with cardiac defects & hypocalcemia; HIV infection in infants can cause opportunistic infections but usually has maternal risk factors and positive HIV PCR.

1. Hematopoietic stem cell transplant (HSCT) as early as possible (ideally in the first 3–4 months) offers cure by reconstituting the immune system.
2. Gene therapy has been successful in some SCID types (e.g. IL2RG X-linked SCID, ADA-SCID); ADA deficiency can also be treated with PEG-ADA enzyme replacement.
3. Supportive care: IVIG (IV immunoglobulin) to provide antibodies, prophylactic antimicrobials (e.g. TMP-SMX for PCP, antifungals), and strict infection precautions (sterile environment, no live vaccines).
4. If SCID is suspected or confirmed, isolate the infant and avoid exposures; even maternal CMV (from breast milk) can be dangerous if donor milk isn't screened.

• The classic "bubble boy" scenario refers to SCID infants kept in sterile isolation.
• Maternal IgG wanes by 3–6 months – that's when SCID manifests with infections (early newborn period is often protected).

• Severe reactions to live vaccines (e.g. disseminated BCG or rotavirus infection) in an infant – red flag for SCID, requires immediate evaluation.
• Infant with persistent thrush, failure to thrive, and recurrent infections – evaluate immediately for SCID (it's a medical emergency).

1. Newborn screening (TREC) is low or infant has lymphopenia/infections → suspect SCID and urgently consult immunology.
2. Confirm with flow cytometry (absent T cells, ± B/NK cells) and genetic testing for specific mutation.
3. Begin protective measures: reverse isolation, no live vaccines, broad prophylaxis (antibacterials, antivirals, antifungals, PCP prophylaxis, IVIG).
4. Definitive treatment: prompt HSCT from a matched donor (or gene therapy if available) for long-term cure.

• Infant (~4–6 months old) with oral thrush, chronic diarrhea, Pneumocystis pneumonia, and no thymic shadow on CXR → think SCID (X-linked IL2RG deficiency).
• Newborn develops disseminated infection from a live vaccine (e.g. BCG or rotavirus) → suspect SCID (inability to handle attenuated pathogens).