T-B+ severe combined immunodeficiency due to CD3delta/CD3epsilon/CD3zeta

T-B+ severe combined immunodeficiency (SCID) due to CD3delta, CD3epsilon, or CD3zeta deficiency is a group of rare, life-threatening primary immunodeficiency disorders caused by mutations in the genes encoding components of the CD3 signaling complex of the T-cell receptor (CD3D, CD3E, or CD247/CD3Z, respectively). These defects impair T-cell development and function while leaving B-cell and natural killer (NK) cell numbers relatively preserved, resulting in the T-B+NK+ SCID immunophenotype (though NK cell numbers may vary). Because functional T cells are essential for adaptive immunity, affected infants typically present in the first months of life with severe, recurrent, and opportunistic infections including pneumonia, chronic diarrhea, oral candidiasis, and failure to thrive. Without treatment, the condition is fatal in early childhood. The CD3 complex (composed of CD3 delta, epsilon, gamma, and zeta chains) is critical for surface expression of the T-cell receptor and for intracellular signal transduction following antigen recognition. Deficiency of any one of these chains leads to a profound block in T-cell maturation in the thymus. CD3 delta deficiency (OMIM #608971) and CD3 epsilon deficiency (OMIM #615615) typically cause a complete absence of circulating T cells, while CD3 zeta deficiency (OMIM #610163) may present with reduced but not entirely absent T cells and can have a somewhat milder or later presentation in some cases. The definitive treatment for this group of disorders is hematopoietic stem cell transplantation (HSCT), which can reconstitute the immune system if performed early, ideally before the onset of severe infections. Early identification through newborn screening programs that detect T-cell receptor excision circles (TRECs) has improved outcomes by enabling prompt diagnosis and treatment. Gene therapy is under investigation as a potential future therapeutic option. Supportive care includes infection prophylaxis with antimicrobials, immunoglobulin replacement therapy, and strict avoidance of live vaccines. Affected patients should be managed in specialized immunology centers.

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