What is Human Leukocyte Antigen (HLA)?

What is HLA (Human Leukocyte Antigen)?

HLA (Human Leukocyte Antigen) is a group of highly polymorphic genes located on chromosome 6 that encode cell surface proteins responsible for presenting antigenic peptides to T cells, thereby regulating immune responses and determining tissue compatibility for transplantation. 1

Genomic Location and Structure

The HLA system is part of the Major Histocompatibility Complex (MHC) located on the short arm of chromosome 6 (6p21.31), comprising over 200 genes with immune functions 1. The locus is divided into three main regions:

• Class I region: Contains HLA-A, HLA-B, and HLA-C genes
• Class II region: Contains HLA-DPA1, HLA-DPB1, HLA-DQA1, HLA-DQA2, HLA-DQB1, HLA-DQB2, HLA-DRA, HLA-DRB1, HLA-DRB2, HLA-DRB3, HLA-DRB4, and HLA-DRB5 genes
• Class III region: Contains complement genes and other immune-related genes 1

Molecular Function

HLA Class I Molecules

MHC class I molecules are expressed on nearly all nucleated somatic cells and present intracellular peptides (self-antigens or cytosolic pathogens) to CD8+ T cells 1. The class I molecule consists of:

• An α-chain encoded in the MHC class I region
• A β2-microglobulin chain encoded on chromosome 15
• A closed-ended antigen-binding groove that restricts peptide size to 8-9 amino acids 2

HLA Class II Molecules

MHC class II molecules are expressed primarily on antigen-presenting cells and present extracellular peptides to CD4+ T cells 1. The class II molecule consists of:

• Both α- and β-chains encoded within the MHC region
• An open-ended antigen-binding groove that accommodates longer peptides (12-28 amino acids) 2

Clinical Significance

Disease Associations

The HLA locus is associated with more complex diseases than any other locus in the human genome, and in many diseases, HLA explains more heritability than all other known loci combined 1. Disease mechanisms include:

• Rheumatoid arthritis: HLA-DRB1 polymorphisms alter autoantigen presentation or increase autoreactive T cells during thymic selection 1
• Psoriasis: HLA-C*06:02 allele increases CD8+ T-cell-mediated inflammatory reactions 1
• Type 1 diabetes: Specific HLA alleles confer susceptibility 1

Transplantation Biology

HLA typing is essential for assessing donor-recipient immune compatibility in organ and stem cell transplantation 3. Key concepts:

• Each individual inherits two sets of HLA antigens (one from each parent)
• HLA-A, -B, and -DR constitute the "haplotype" 3
• Among siblings: 25% chance of complete match, 25% chance of complete mismatch, 50% chance of half-match 3
• Accurate HLA typing, especially for HLA-DR genes, is essential to ensure safety and prognosis of organ transplantation 1

Pharmacogenetics

Specific HLA alleles are significantly associated with idiosyncratic adverse drug reactions (drug hypersensitivity reactions) 4. HLA genetic testing shows promise in predicting and preventing severe drug reactions.

Unique Genetic Characteristics

The HLA system exhibits several distinctive features 1:

• Extreme polymorphism: The most polymorphic genes in the human genome
• Codominant expression: Both maternal and paternal alleles are expressed
• Haplotype inheritance: Alleles are inherited as linked blocks
• Long-range linkage disequilibrium: Variants across the entire MHC region are correlated
• Population-specific diversity: HLA allele frequencies vary dramatically across ethnic groups
• Pathogen-driven natural selection: Evolutionary pressure has shaped HLA diversity

Common Pitfalls

The highly polymorphic nature of HLA genes creates challenges:

• Traditional probe-based genotyping is difficult and costly 1
• Cross-reactions in serologic testing complicate interpretation 5
• Population-specific diversity requires multi-ancestry reference panels for accurate typing 1