Cross presentation is the presentation of peptides derived from exogenous acquired antigen in association with HLA class I in professional antigen presenting cell. Cross-priming is the sensitization of CD8 T cells by dendritic cells to such antigens. Cross-priming plays an important role during priming of antitumours CD8 T cells as well as priming CD8 respons to pathogens which do not directly infect dendritic cells. Although various cell types are able to cross-present under certain condition. Dendritic cells are the most important cress-presenting cells in vivo.