Cytotoxic T-cell (CTL) once activated it initiate killing of target cells by attaching to it. After interaction with a target cell, it undergoes conjugate formation with the help of calcium. The killing action is performed by cytotoxic substances contained in granules of T-cells.

Perforin and granzyme are the cytotoxic substances present in granules of CTL. Perforin involves in the formation of pores on the target cell while granzyme performs apoptosis by direct activation of apoptosis.