Brief me about HLA
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HLA (Human Leukocyte Antigen) - A Complete Brief
What Is the HLA System?
The human leukocyte antigen (HLA) complex is the human version of the major histocompatibility complex (MHC) - a set of genes located on chromosome 6p21 spanning approximately 3 Mb. It is the most polymorphic locus in the human genome, with over 200 genes and well over 28,000 alleles identified across the Class I and Class II HLA genes.
HLA proteins are expressed on the surface of nearly all nucleated cells, where they bind and display peptide fragments to circulating immune cells. If those peptides are recognized as foreign (viral, bacterial, or modified self), the immune system responds accordingly.
- Thompson & Thompson Genetics and Genomics, 9th ed.
- Rheumatology, 2-Volume Set (Elsevier, 2022)
Gene Organization: Three Classes
| Class | Genes | Expression | Function |
|---|---|---|---|
| Class I | HLA-A, HLA-B, HLA-C | Nearly all nucleated cells (not erythrocytes or trophoblasts) | Present endogenous (cytosolic) peptides to CD8+ cytotoxic T cells |
| Class II | HLA-DP, HLA-DQ, HLA-DR | Antigen-presenting cells (dendritic cells, macrophages, B cells) | Present exogenous (internalized) peptides to CD4+ T helper cells |
| Class III | C2, C4, Factor B, TNF-alpha, lymphotoxin, heat shock proteins | Various | Immune regulation, complement - the densest gene region in the human genome (58 expressed genes) |
- Rheumatology, 2-Volume Set (Elsevier, 2022)
- Tietz Textbook of Laboratory Medicine, 7th ed.
Antigen Presentation Pathways
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Endogenous pathway (Class I): Intracellular proteins (e.g., from replicating viruses) are degraded in the cytosol via the proteasome. Resulting peptides are transported into the endoplasmic reticulum, loaded onto MHC Class I molecules, and displayed on the cell surface for CD8+ T cells to inspect.
-
Exogenous pathway (Class II): Extracellular antigens are taken up by phagocytosis or endocytosis, degraded in lysosomes, and loaded onto MHC Class II molecules, then displayed for CD4+ T helper cells.
-
Harrison's Principles of Internal Medicine, 22nd ed.
Polymorphism and Nomenclature
HLA is extraordinarily polymorphic. HLA-B alone has 7,562 identified alleles. The old serologic system (e.g., "B27," "DR4") has been replaced by DNA-sequence-based nomenclature:
Format:
HLA-[Locus]*[Field 1]:[Field 2]:[Field 3]:[Field 4][Suffix]- Locus: A, B, C, DRB1, DQA1, DQB1, DPA1, DPB1, etc.
- * confirms molecular typing was used
- Field 1: Allele group
- Field 2: Specific allele (differs by ≥1 amino acid in mature protein)
- Field 3: Synonymous coding substitution
- Field 4: Non-coding region difference
- Suffixes: N (null), L (low expression), S (secreted/soluble), C (cytoplasmic only), A (aberrant), Q (questionable)
Example:
HLA-B*57:01 differs from HLA-B*57:03 by only two base substitutions but has different clinical implications.What was serologically a single "B27" allele is now HLA-B*2701, HLA-B*2702, etc.
- Brenner and Rector's The Kidney, 2-Volume Set
- Tietz Textbook of Laboratory Medicine, 7th ed.
HLA and Disease Associations
HLA class II is associated with more than 100 diseases - autoimmune, inflammatory, infectious, and even some cancers. The first established genetic association with lupus was the HLA region, found in 1971.
Key associations (from Robbins Pathologic Basis of Disease and Quick Compendium of Clinical Pathology):
| Disease | HLA Association | Odds Ratio (approx.) |
|---|---|---|
| Ankylosing spondylitis | HLA-B27 (mainly B*2705, B*2702) | 100-200x |
| Rheumatoid arthritis (anti-CCP+) | DRB1 shared epitope (positions 70-74) - 1 allele: 4x; 2 alleles: 12x | 4-12x |
| Type 1 Diabetes | DRB1*0301/0401 heterozygotes; protective: DQB1*0602 | up to 35x |
| Celiac disease | DQ2 (DQA1*0501-DQB1*0201) and DQ8 | 7x |
| Multiple sclerosis | DRB1*1501, DQB1*0602 | ~3x |
| SLE | DRB1*0301, DRB1*1501 | 1.3-2x |
| Narcolepsy | DQ6 (DQB1*0602) | very high |
| Hereditary hemochromatosis | HLA-A3 | - |
| 21-hydroxylase deficiency | HLA-C4 | - |
| Behcet disease | HLA-B51 | - |
Important caveat: HLA alleles contribute to disease risk but are not sufficient causes. Most HLA-B27 carriers never develop ankylosing spondylitis.
HLA and Drug Hypersensitivity (Pharmacogenomics)
The HLA-B gene, due to its high polymorphism, has the strongest pharmacogenomic significance. Certain HLA variants predispose individuals to severe cutaneous adverse reactions (SCAR) including:
- Stevens-Johnson Syndrome (SJS): <30% body surface area affected
- Toxic Epidermal Necrolysis (TEN): >30% body surface area; mortality up to 40%
These are Type B adverse drug reactions and occur rarely in the general population (1-6 per 10,000) but are ~10x more common in Asian populations.
Key pharmacogenomic HLA associations (FDA-mandated testing):
| Drug | HLA Allele | Reaction |
|---|---|---|
| Abacavir (HIV) | HLA-B*57:01 | Hypersensitivity syndrome |
| Carbamazepine (epilepsy) | HLA-B*15:02 (Asian populations) | SJS/TEN |
| Allopurinol (gout) | HLA-B*58:01 | SJS/TEN |
Note: A surrogate marker rs2395029 in HCP5 has 99.9% predictive value for HLA-B*57:01, but false negatives can occur due to copy number variation.
- Tietz Textbook of Laboratory Medicine, 7th ed.
HLA in Transplantation
HLA matching is essential in solid organ and hematopoietic stem cell transplantation. Mismatches drive:
- T cell-mediated rejection (direct pathway: donor APCs presenting intact HLA to recipient T cells; indirect pathway: recipient APCs processing donor HLA peptides)
- Antibody-mediated rejection (donor-specific antibodies, or DSA, against mismatched HLA antigens)
For clinical transplantation, HLA typing is typically reported at fields 1-2. DQ and DP antigens should specify both alpha and beta chains because the immunologic molecule is a heterodimer.
- Brenner and Rector's The Kidney, 2-Volume Set
- Bailey and Love's Short Practice of Surgery, 28th ed.
HLA Haplotypes and Linkage Disequilibrium
HLA alleles at the different Class I and Class II loci on one chromosome form a haplotype - and due to extensive linkage disequilibrium (LD), certain allele combinations are inherited together as a unit far more frequently than expected by chance. This has major implications for:
- Disease association studies (reported associations may reflect LD with true causal alleles)
- Transplant matching (haplotype-matched sibling donors are ideal)
- Population genetics (different ancestral groups have distinct haplotype frequencies)
Summary
HLA is the most polymorphic region of the human genome, encoding cell surface molecules that are the master regulators of adaptive immune recognition. Its three classes serve distinct but complementary roles in immune surveillance. Its extreme polymorphism explains its central role in transplant rejection, autoimmune susceptibility, and drug hypersensitivity. Understanding HLA is foundational to immunology, clinical medicine, pharmacogenomics, and transplantation science.
Detailed about immune response
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