[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) › MHC/HLA class I and II [Immunology](https://durgical.com/en/immunology/) · [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) # MHC/HLA class I and II - Molecule Display molecules on our cells that show protein fragments to T cells; in humans they are called HLA and differ from person to person. Updated 2026-11-01 ## Key points - Class I: all nucleated cells, endogenous peptides, CD8 T cells, beta-2 microglobulin. - Class II: antigen-presenting cells, exogenous peptides, CD4 T cells, invariant chain. - Rule of 8: CD4 times class II = 8, CD8 times class I = 8. - HLA-B27 with spondyloarthropathies; DR3/DR4 with type 1 diabetes; DQ2/DQ8 with celiac disease. - HLA genes are on chromosome 6 and are the main barrier in transplantation. ## What it is The major histocompatibility complex (MHC), called HLA in humans and encoded on chromosome 6, makes surface molecules that hold peptides for T cells to inspect. Class I (HLA-A, -B, -C) is on almost all nucleated cells and shows intracellular peptides to CD8 T cells; class II (HLA-DP, -DQ, -DR) is on antigen-presenting cells and shows extracellular peptides to CD4 T cells. HLA genes are highly polymorphic, which matters for transplant matching and for disease associations. ## Key facts - HLA genes are inherited as haplotypes, one from each parent; a sibling has a one-in-four chance of being HLA-identical. - NK cells kill cells that lose class I, a common viral and tumor escape strategy. - Interferon-gamma increases expression of both MHC classes. ## MHC class I versus class II | Feature | Class I | Class II | | --- | --- | --- | | Genes | HLA-A, HLA-B, HLA-C | HLA-DP, HLA-DQ, HLA-DR | | Where it is expressed | All nucleated cells and platelets (not mature red cells) | Professional antigen-presenting cells: dendritic cells, macrophages, B cells | | Structure | One alpha chain with three domains plus beta-2 microglobulin | Alpha and beta chains, both crossing the membrane | | Peptide source | Intracellular (cytosolic) proteins, for example viral | Extracellular proteins taken up into endosomes | | Loading | Proteasome, TAP, endoplasmic reticulum | Endosome; invariant chain and CLIP exchanged with help from HLA-DM | | T cell that reads it | CD8 cytotoxic T cell | CD4 helper T cell | ## Clinical notes - HLA-B27: ankylosing spondylitis, reactive arthritis, psoriatic arthritis and arthritis of inflammatory bowel disease. - HLA-DR3 and HLA-DR4: type 1 diabetes. - HLA-DQ2 and HLA-DQ8: celiac disease. - HLA-DR4 (shared epitope): rheumatoid arthritis. - HLA-B*57:01: hypersensitivity to abacavir, so it is tested before that drug is used. - Bare lymphocyte syndrome (MHC class II deficiency): very few CD4 T cells and combined immunodeficiency. ## Memory aid Class I is the window of every house (shows what is cooking inside to the CD8 police); class II is the shop window of the antigen-presenting cell (shows what it bought outside to the CD4 managers). ## Test yourself Think of the answer, then tap the question to see it. ## Which T cell recognizes peptides presented on MHC class I? The CD8 cytotoxic T cell. ## Which cells normally express MHC class II? Professional antigen-presenting cells: dendritic cells, macrophages and B cells (thymic epithelium also expresses it). ## Which HLA allele is associated with ankylosing spondylitis? HLA-B27. ## Which protein forms the light chain of MHC class I? Beta-2 microglobulin. ## Which HLA molecules are associated with celiac disease? HLA-DQ2 and HLA-DQ8. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/immunology/mhc-hla-class-i-and-ii/) ## Related - [Antigen processing and presentation](https://durgical.com/en/immunology/antigen-processing-and-presentation/) - [T-cell development: positive and negative selection](https://durgical.com/en/immunology/t-cell-development-positive-and-negative-selection/) - [T-cell activation: two signals and superantigens](https://durgical.com/en/immunology/t-cell-activation-two-signals-and-superantigens/) - [HLA associations with disease](https://durgical.com/en/immunology/hla-associations-with-disease/) - [Transplant rejection: hyperacute, acute, chronic](https://durgical.com/en/immunology/transplant-rejection-hyperacute-acute-chronic/) - [CD4⁺ helper T cell](https://durgical.com/en/immunology/cd4-helper-t-cell/) - [CD8⁺ cytotoxic T cell](https://durgical.com/en/immunology/cd8-cytotoxic-t-cell/) - [Natural killer cell](https://durgical.com/en/immunology/natural-killer-cell/) ## Related conditions - [Ankylosing spondylitis](https://durgical.com/en/conditions/ankylosing-spondylitis/) - [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/) - [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/) - [Rheumatoid arthritis](https://durgical.com/en/conditions/rheumatoid-arthritis/) [← All of immunology](https://durgical.com/en/immunology/) Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given. Sources: [18.2 Major Histocompatibility Complexes and Antigen-Presenting Cells](https://openstax.org/books/microbiology/pages/18-2-major-histocompatibility-complexes-and-antigen-presenting-cells) (OpenStax (Microbiology), 2016); [21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-3-the-adaptive-immune-response-t-lymphocytes-and-their-functional-types) (OpenStax (Anatomy and Physiology 2e), 2022). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/immunology/mhc-hla-class-i-and-ii/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/mhc-hla-class-i-and-ii/