Phosphorylation of serine and threonine residues is conserved during MHC class I and MHC class II antigen processing. Phosphopeptides are thus displayed on the surface of cells.
During the process of MHC class I and MHC class II antigen processing, the phosphorylation of serine and threonine residues persists. These phosphorylated peptides are presented on the cell's exterior.