MHCpresentations

MHC presentations refers to the process by which peptide antigens are displayed on the surface of cells by major histocompatibility complex (MHC) molecules for recognition by T cells. In humans, MHC genes are called human leukocyte antigen (HLA) genes, and their extensive polymorphism shapes which peptides can be bound and presented to the immune system.

Two main classes operate in distinct pathways. MHC class I molecules present endogenous peptides, derived from proteins synthesized inside the cell, to CD8+ T cells. Proteins are degraded by the proteasome, and peptide fragments are transported into the endoplasmic reticulum by TAP transporters, where they are loaded onto heavy-chain MHC I molecules with the help of chaperones and the peptide-loading complex before migrating to the cell surface. MHC class II molecules present exogenous peptides, from proteins taken up by endocytosis, to CD4+ T cells. Invariant chain initially blocks the binding groove in the ER; within endosomes the chain is degraded to CLIP, and peptides are loaded with the help of HLA-DM (or H2-DM in mice) before the MHC II-peptide complex reaches the cell surface. Class II peptides are typically longer, around 13–18 amino acids, compared with class I peptides (~8–10 aa).

Cross-presentation is a specialized process wherein certain antigen-presenting cells, notably dendritic cells, present extracellular antigens on MHC I to activate CD8+ T cells. MHC presentation is central to immune surveillance, vaccine responses, transplantation compatibility, autoimmunity, and cancer immunotherapy, driven by the diversity and expression patterns of HLA alleles and peptide repertoires.