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Cross-presentation is the ability of certain professional antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells. Cross-priming, the result of this process, describes the stimulation of naive cytotoxic CD8 T cells into activated cytotoxic CD8 T cells. This process is necessary for immunity against most tumors and against viruses that infect dendritic cells and sabotage their presentation of virus antigens. Cross presentation is also required for the induction of cytotoxic immunity by vaccination with protein antigens, for example, tumour vaccination.
Cross-presentation is of particular importance, because it permits the presentation of exogenous antigens, which are normally presented by MHC II on the surface of dendritic cells, to also be presented through the MHC I pathway. The MHC I pathway is normally used to present endogenous antigens that have infected a particular cell. However, cross presenting cells are able to utilize the MHC I pathway in order to remain uninfected, while still triggering an adaptive immune response of activated cytotoxic CD8+ T cells against infected peripheral tissue cells.