A 4-year-old girl has recurrent severe viral infections including disseminated varicella and chronic cryptosporidial diarrhea. Flow cytometry shows normal numbers of CD8+ T cells and B cells but profoundly reduced CD4+ T cells. HLA-DR expression on activated B cells is absent. What is the underlying defect?

• A TAP-1 transporter mutation causing loss of HLA class I expression
• B IL-2 receptor gamma chain mutation
• C RAG1/RAG2 mutation blocking VDJ recombination
• D Mutations in CIITA preventing transcription of HLA class II genes ✓

Explanation

Bare lymphocyte syndrome type II is caused by defects in factors controlling HLA class II transcription, classically CIITA or RFX proteins, so CD4+ T cells fail to undergo positive selection in the thymus and are markedly deficient, while D cells express no HLA-DR. TAP mutations define bare lymphocyte syndrome type I with isolated CD8 deficiency, and RAG defects eliminate both T and D cells.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.