Table 1.

Immunophenotyping in severe combined immune deficiency (SCID) with associated genetic defects.

	Lymphocyte phenotype	Associated genetic defects	Comments
I.	T–B–NK– SCID	ADA, PNP	Accumulation of toxic metabolites inhibits DNA synthesis and repair and leads to severe lymphopenia (7).
II.	T–B-NK+ SCID	RAG1, RAG2, DCLRE1C, LIG4, NHEJ1	Defects in somatic recombination result in decreased or absent T and B lymphocytes (8).
III.	T–B+NK– SCID	IL2RG, JAK3	T-B+NK- SCID results from defects in common gamma chain that is required for normal development of T and NK cells (9). Analysis of surface expression of CD132 may also help in identifying the defect (10).
IV.	T–B+NK+ SCID	IL7 R, CD3δ, CD3ε and CD3ζ	Reduced surface expression of CD127 on T cells can help in classifying SCID (11).
V.	Omenn syndrome	RAG1, RAG2, DCLRE1C, ADA, LIG4, IL2RG, IL7R, DiGeorge syndrome	Reduced naïve T cells (CD3+45RA+45RO−), elevated memory T cells (CD3+45RA-45RO+), and increased expression of HLA DR on T lymphocytes are noted in Omenn syndrome (12). T cell receptor Vβ repertoire analysis shows skewed Vβ usage indicating oligoclonality (13).