MHC Class II molecules function by presenting processed peptide antigens to Helper T cells (CD4+ T cells). When an APC engulfs a pathogen, the pathogen is broken down in the endosome-lysosome system. The resulting peptides are then loaded onto MHC Class II molecules within these compartments. The MHC Class II-peptide complex is transported to the cell surface, where it can be recognized by the T cell receptor (TCR) on CD4+ T cells, initiating an immune response.
