A child has severe recurrent viral, fungal, and pyogenic infections from early infancy. Flow cytometry shows normal numbers of CD4+ and CD8+ T cells and B cells, but T cells fail to respond to antigen presented by APCs. HLA-DR expression on monocytes is absent while HLA-DP and HLA-DQ are also undetectable. What is the underlying defect?

• A Mutation in TAP1 transporter protein
• B Defective CIITA transcription factor causing MHC class II deficiency ✓
• C Adenosine deaminase deficiency
• D IL-2 receptor gamma chain mutation

Explanation

Bare lymphocyte syndrome type II is MHC class II deficiency caused by defects in transcription factors such as CIITA or RFX proteins that regulate HLA-DP, DQ, and DR expression. CD4+ T cells fail to develop properly because positive selection in the thymus requires class II recognition, so cellular and humoral immunity both collapse despite normal lymphocyte counts. TAP1 mutation causes class I deficiency, ADA deficiency causes true SCID with lymphopenia, and IL-2R gamma mutation causes X-linked SCID, all inconsistent with preserved counts and absent class II expression.

Reference: Abbas, Basic Immunology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.