MHC-class I is expressed ubiquitously and, for the most part, displays peptides derived from intracellular sources, with the notable exception of certain specialized cell types such as CD8αα+ DCs that have the ability to load proteins derived from extracellular sources onto MHC-class I. This specialized pathway, termed crosspresentation, allows priming of cytotoxic CD8+ T-cells by professional antigen-presenting cells that are not directly infected.