Flow cytometry of peripheral blood lymphocytes from an infant with recurrent severe candidiasis and Pneumocystis jirovecii pneumonia reveals normal numbers of CD8 positive T cells and B cells but profoundly reduced CD4 positive T cells. Further testing shows complete absence of HLA DP, DQ, and DR molecules on the patient's B cells. The underlying defect is:

• A Mutation of transcription factors regulating MHC class II gene expression, such as CIITA ✓
• B Failure of thymic development of medullary epithelium
• C Defective expression of the common gamma chain cytokine receptor
• D Absence of the TAP 1 transporter peptide loader

Explanation

This is bare lymphocyte syndrome, the MHC class II deficiency. Mutations in genes controlling class II transcription, including CIITA (the class II transactivator) and RFX family factors, prevent expression of HLA DP, DQ, and DR on all cells. Without class II molecules, CD4 positive T cells cannot be positively selected in the thymus, so they are markedly reduced despite normal CD8 counts, producing combined cellular and humoral failure. TAP defects impair class I loading and would reduce CD8 cells instead.

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.