Thus, in terms of induction of immunity, there is an influence of the site and procedure used for injection, with muscle and skin cells clearly able to act as antigen depots but unable to prime the immune response. It is likely that cross-presentation from these sites to APCs is the major route to priming [26] , but there is also evidence for direct transfection of APCs, especially when delivery is to skin sites through a gene gun [25] . The host-synthesized antigen is then processed and presented by APCs in the context of both MHC I and MHC II.