![cell surface markers first aid 2017 cell surface markers first aid 2017](https://pubs.rsc.org/image/article/2021/cs/d0cs00152j/d0cs00152j-f2_hi-res.gif)
![cell surface markers first aid 2017 cell surface markers first aid 2017](https://www.mdpi.com/microorganisms/microorganisms-08-00504/article_deploy/html/images/microorganisms-08-00504-g001-550.jpg)
![cell surface markers first aid 2017 cell surface markers first aid 2017](https://media.springernature.com/full/springer-static/image/art%3A10.1038%2Fs41598-020-61356-w/MediaObjects/41598_2020_61356_Fig1_HTML.png)
Although there is a vast amount of biochemical and structural information, the mechanism of the catalyzed peptide exchange for MHC class I and class II proteins still remains controversial, and it is not well understood why certain MHC allelic variants are more susceptible to peptide editing than others. Additionally, two peptide editors-tapasin for class I and HLA-DM for class II-contribute to the shaping of the presented peptidome by favoring the binding of high-affinity antigens. Once processed, the peptide repertoire presented by MHC proteins largely depends on structural features of the binding groove of each particular MHC allelic variant. The prolonged interaction between a T cell receptor and specific pMHC complexes, after an extensive search process in secondary lymphatic organs, eventually triggers T cells to proliferate and to mount a specific cellular immune response. Prior to presentation, peptides need to be generated from proteins that are either produced by the cell’s own translational machinery or that are funneled into the endo-lysosomal vesicular system.