A 14-month-old girl has recurrent severe viral, bacterial, and fungal infections. Flow cytometry shows normal CD8+ T cells and B cells but profoundly reduced CD4+ T cells. HLA class I molecules are expressed normally on lymphocytes but HLA class II molecules are absent. What is the diagnosis?

• A Bare lymphocyte syndrome type II due to defective transcription factors controlling MHC class II genes ✓
• B DiGeorge syndrome due to 22q11.2 microdeletion
• C MHC class I deficiency due to TAP1 or TAP2 mutation
• D Adenosine deaminase deficiency causing autosomal recessive SCID

Explanation

Bare lymphocyte syndrome type II is caused by mutations in transcription factors such as CIITA or RFX genes that control MHC class II expression. Without class II molecules, positive selection of CD4+ T cells in the thymus fails, so CD4 cells are absent while CD8 cells are preserved. Class I expression remains normal, excluding TAP defects. DiGeorge syndrome reduces both T cell subsets, and ADA deficiency causes profound combined lymphopenia affecting both lineages.

Reference: Abbas, Cellular and Molecular Immunology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.