Lonely MHC molecules seeking immunogenic peptides for meaningful relationships

The association between antigenic peptides and MHC class II molecules represents a critical event in the initiation of the immune response to extracellular antigens. Understanding the molecular basis of antigen processing requires the characterization of the intracellular compartments, or 'singles bars', in which immunogenic peptides are generated and loaded onto class II molecules. In the past year, something of a breakthrough occurred with the identification of specialized compartments that host antigen processing and/or peptide loading, designated 'MHC class II compartment' and 'class II vesicles'. It is becoming increasingly clear, however, that these compartments are themselves heterogeneous and not always distinct from conventional endosomes and lysosomes.