 A non classical MHC-II molecule (HLA-DO and HLA-DM) catalyses the exchange of part of the CD74 (CLIP peptide) with the peptide antigen.Peptide-MHC-II complexes (pMHC-II) are transported to the plasma membrane and the processed antigen is presented to the helper T cells in the lymph nodes.APCs undergo a process of maturation while migrating, via chemotactic signals, to lymphoid tissues, in which they lose the phagocytic capacity and develop an increased ability to communicate with T-cells by antigen-presentation.