Several types including defective IL-2R gamma chain (most common, X-linked), adenosine deaminase deficiency (autosomal recessive)

- Failure to thrive- Chronic bacterial diarrhea- Chronic candidiases (eg, thrush)- Recurrent viral, bacterial, fungal, and protozoal infections

Findings:- ↓ T-cell receptor excision circles (TRECs)- Absence of thymic shadow (CXR), germinal centers (lymph node biopsy), and T cells (flow cytometry)

Treatment: - Avoid live vaccines- Give antimicrobial prophylaxis and IVIG- Bone marrow transplant curative (no concern for rejection)