Dendritic cells in peripheral immune organs are quiescent sentinels that continuously survey their surroundings by fluid phase uptake and endocytosis. Dendritic cells become activated when pathogen-derived fragments are detected. Engulfed material is now processed into peptides and lipids for incorporation into peptide/MHC or lipid/CD1 complexes, for display to MHC- and CD1-restricted T cells respectively. The presentation of antigen to T cells is regarded the initiation of immune responses. Much of what we know about the intracellular antigen processing and loading has been obtained from biochemical and cell biological analysis of B cell lines, and to a more limited extent, of cultured dendritic cells.New methodology now makes it possible to visualize the uptake of fluorescent antigen by live Class II MHC-positive dendritic cells in situ without the need for fixation or staining, by analysis of dendritic cells in the epidermis of mice that carry fusion proteins composed of Class II MHC molecules and green fluorescent protein. We will study the protein and lipid antigen-presentation pathways in skin-derived dendritic cells. We propose to study the importance of antigen entry into specialized endosomal compartments in prototypic immature dendritic cells in situ, by analysis of Langerhans cells in epidermis.The loading of lipid antigens in late endosomal compartments in primary cells has not been explored to date, due to the unavailability of suitable mouse models. We have very recently succeeded in the generation of novel fluorescent knock-in mice in which we shall study the intracellular trafficking routes of membrane protein CD1d.