MHC Class II Deficiency
An autosomal recessive combined immunodeficiency caused by defects in transcription factors regulating MHC class II expression, resulting in impaired CD4+ T-cell development and antigen presentation.
Full Definition
MHC class II deficiency, also termed bare lymphocyte syndrome type II, arises from mutations in the genes encoding CIITA, RFXANK, RFX5, or RFXAP—transcription factors required for constitutive and inducible MHC class II expression on antigen-presenting cells. The absence of HLA-DR, HLA-DP, and HLA-DQ molecules on antigen-presenting cells leads to defective thymic positive selection of CD4+ T cells, resulting in a CD4+ lymphopenia despite relatively preserved CD8+ T cells. Affected children, often of North African or Middle Eastern descent, present in infancy with severe infections and protracted diarrhoea. Editors must render 'MHC class II' with a space before the Roman numeral 'II' and should not confuse this entity with MHC class I deficiency, which has a distinct genetic basis and milder clinical phenotype.
Usage
Usage note: Always include the space between 'class' and 'II'. Do not use 'MHC-II deficiency' with a hyphen. Distinguish from MHC class I deficiency (bare lymphocyte syndrome type I), which affects CD8+ T cells and presents with vasculitis rather than profound infections.
In Context
- "MHC class II deficiency was confirmed by absent HLA-DR expression on peripheral blood monocytes, with subsequent sequencing revealing a homozygous RFXANK mutation." — Paediatric immunology diagnostic report
- "The manuscript conflated MHC class I and MHC class II deficiency; the editor restructured the paragraph to clarify their distinct transcriptional mechanisms." — Peer-review correction