Figure 1.

Classical antigen presenting by MHC class II molecule. MHC class II is an important part in antigen presenting process especially in presentation of exogenous antigens. APCs can take up antigens through phagocytosis, pinocytosis and other ways. Protein antigens will go into the endosomes. The α chain and β chain of MHC class II are dimerized in endoplasmic reticulum and combine with Ia-associated invariant chain (Ii) to form (αβIi)₃ as nonamer. The nonamer will be sent to Golgi apparatus and then form the MHC class II compartment (MIIC). In MIIC the Ii will be decomposed, but class II-associated invariant chain peptide (CLIP) will still be preserved. The endosome merges with MIIC, CLIP will be removed with assistance of HLA-DM (not shown in figure) and the peptide of antigen will be loaded on MHC class II subsequently. Then it can be presented to CD4⁺ T cells to activate them and trigger the latter reactions.