Presentation of antigens by MHC class II molecules: getting the most out of them

The function of MHC class II molecules is to bind peptides derived from ant igens that access the endocytic route or antigen presenting cells and displ ay them on the plasma membrane for recognition by CD4' T cells. Formation o r the MHC II-peptide complexes entails the confluence of the antigens and t he MHC II molecules in the same compartments of the endocytic route. There, both the antigens and the MHC II molecules undergo a series of orchestrate d changes that involve proteases, other hydrolases and chaperones, culminat ing in the generation of a wide repertoire of MHC II-peptide combinations. All the events that lead to formation of MHC II-peptide complexes show a co nsiderable degree of flexibility; this lack of strict rules is advantageous in that it provides T cells with the maximum amount of information, ensuri ng that pathogens do not go undetected. (C) 2001 Published by Elsevier Scie nce Ltd.