Cytotoxic T cell (CTL) covers several subtypes, which are CD8+, CD4 and CD4-CD8-. CTL derives from T cell repertoire in lymphoid hematopoietic stem cells. It matures in thymus and is activated in peripheral lymphoid tissues. Effector CTL kills the target cells by 2 ways. One is apoptotic effect mediated by FasL-Fas pathway and the other one is cytolytic effect mediated by granzymes. CTL has aroused great attention due to its significance in anti-tumor and anti-virus.