An NK cells and PBMC, both having increased cytotoxicity are provided. Particularly, NK cells and PBMC which have exogenous mitochondria introduced thereinto potentiate the immune system of the human body to enhance a therapeutic effect on infectious diseases or cancer. Therefore, the NK cells and PBMC can be used in a composition for prevention or treatment of infectious diseases and cancer. Specially, autogenous NK cells and PBMC guarantee stability without the incurrence of an immune reaction, and thus would be expected to have high commercial activity.