MHC Class I vs Class II- Definition, 15 Differences, Examples

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MHC Class I and Class II differences (mhc i vs mhc ii)

Differences between MHC Class I and Class II

The major differences are:

S.N.

 Characteristics MHC-I molecule

MHC -II molecule
1. Distribution Present on almost all nucleated cells including platelets. Have a restricted tissue distribution and are chiefly found on macrophages, dendritic cells, B cells, and other antigen-presenting cells only.
2. Encoding genes MHC class I proteins are encoded by the HLA-A, HLA-B, and HLA-C genes. MHC Class II proteins are encoded by the genes of the HLA-D region.
3. Nature of antigen presented Antigens presented by MHC class I molecules are of endogenous origin. Antigens presented by MHC class II molecules are derived from extracellular proteins.
4. Antigen Cytosolic proteins; they sample peptides generated within the cell or those that may enter cytosol from phagosomes. Class II molecules sample peptides outside the cell such as lysosomal proteins mostly internalized from extracellular environment.
5. Enzymes involved in peptide generation  Cytosolic proteasome  Endosomal and lysosomal proteases
6. Peptide loading of MHC Endoplasmic reticulum Specialized vesicular compartment
7. Peptide-loading complex Includes the ER transporter associated with antigen processing (TAP1/2), tapasin, the oxidoreductase ERp57, and the chaperone protein calreticulin. Chaperones in ER; invariant chain in ER, Golgi and MHC Class II compartment/Class II vesicle
8. Recognizing co-receptor They are recognized by CD8 co-receptors through the MHC Class I β2 subunit. They are recognized by CD4 co-receptors through β1 and β2 subunits.
9. Receptor T cell Present antigens to CD8+ T cells. Present antigens to CD4+ T cells.
10. Structure MHC class I molecules consist of one membrane-spanning α chain produced by MHC genes, and one β chain produced by the β2-microglobulin gene. MHC class II molecules consist of two membrane-spanning chains, α and β both produced by MHC genes.
11. Building amino acids Possess 8-10 amino acids. Possess 13-18 amino acids.
12. Peptide binding domains  α1 and α2 are peptide binding domains. α1 and β1 are peptide binding domains.
13. Invariant chain Has no invariant chain. Has an invariant chain.
14. Functional effect Presence of abundant antigens target cell for destruction. Presences of foreign antigens induce antibody production.
15. Detection Method Serology Serology and mixed lymphocyte reaction

References

  1. Brooks, G. F., Jawetz, E., Melnick, J. L., & Adelberg, E. A. (2010). Jawetz, Melnick, & Adelberg’s Medical Microbiology. New York: McGraw Hill Medical.
  2. Chelbi, S., Dang, A., & Guarda, G. (2017). Emerging Major Histocompatibility Complex Class I -Realted Functions of NLRC5. Advances in Immunology , 133, 89-119.
  3. https://microbeonline.com/difference-mhc-class-mhc-class-ii-proteins/
  4. https://www.slideshare.net/petermassawe1/similarities-between-class-i-and-class-ii

MHC Class I and Class II differences (mhc i vs mhc ii)

About Author

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Sagar Aryal, PhD

Dr. Sagar Aryal is a microbiologist, researcher, and science communicator from Nepal. He is also co-founder and Research Head of the Department of Natural Products at the Kathmandu Research Institute for Biological Sciences (KRIBS), where he leads research on natural product discovery and microbial biodiversity. Dr. Aryal earned his Ph.D. in Microbiology from Tribhuvan University, Kathmandu, Nepal, in 2025, supported by a DAAD Research Grant and a University Grants Commission (UGC) Nepal Ph.D. Fellowship. His research interests include Actinomycetes, Myxobacteria, microbial taxonomy, and natural products, particularly the discovery of bioactive compounds with potential pharmaceutical applications. He isolated and identified Nocardia nepalensis, a novel bacterial species from Nepal, contributing to understanding microbial diversity in the region. Dr. Aryal has published more than 16 research articles in international journals and contributed three book chapters to established academic publications. He has presented scientific research findings and developed international research collaborations through visits to more than 18 countries across Asia, Europe, North America, and Oceania. From 2023 to 2025, he served as the American Society for Microbiology (ASM) Young Ambassador to Nepal. Since December 2025, he has served as an Editorial Board Member of Scientific Reports, a Nature Portfolio journal. In recognition of his scientific contributions, Dr. Aryal received the NAST Science and Technology Youth Award 2025 from the Nepal Academy of Science and Technology. He previously received the Frs. Locke and Stiller Research Award (LSRA) for the 2017-2018 cycle from St. Xavier's College, Kathmandu, and a Nepal Academy of Science and Technology (NAST) Research Grant for 2017-2018. Alongside his scientific career, Dr. Aryal has more than 15 years of experience in blogging, content writing, and search engine optimization (SEO).