1) MHC II proteins in the ER are assembled with Ii (an invariant protein chain), preventing MHC II from complexing with peptides in the ER.
2) Lysosomes containing MHC II then fuse with phagosomes, forming phagolysosomes
where the Ii and foreign proteins, imported from outside the cell by endocytosis, are digested.
3) The MHC II protein then binds to the digested foreign peptides, and the complex is transported to the cell surface
4) The complex then interacts with TCRs
5) The complex then interacts with the CD4 coreceptor on TH
cells. The TH
cells then release cytokines that act on other cells to promote an immune response.