Cross-presentation is the ability of certain antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells). Relevantly, cross-priming describes the stimulation of naive cytotoxic CD8⁺ T cells by this process. This process is necessary for immunity against most tumors and against viruses that do not readily infect antigen-presenting cells, or impair dendritic cell normal function . It is also required for induction of cytotoxic immunity by vaccination with protein antigens, for example, tumour vaccination.