Antigen presenting cells (APCs) alert the immune system to attack by extracellular organisms; APCs achieve this via internalization, degradation, and display of antigenic fragments on the cell surface by MHC class II molecules. These class II molecules bind to an accessory protein, termed the invariant chain, that ensures proper folding of the molecules. Invariant-chain binding also directs class II molecules to lysosomes, which are probably the most important sites for antigen loading. Endosomes are intermediates in the transport of class-II-invariant chain complexes to antigen-processing compartments, whereas trafficking of class II-peptide complexes to the membrane (and beyond) is less-well understood. Unlike other APCs, dendritic cells alter their capacity to present peptides via MHC class II molecules during differentiation, revealing a complex level of regulated antigen-presentation by this APC subtype.