Perforin ?

Ans. is 'b' i.e., Cytotoxic T cell Cytotoxic T Lymphocyte-Mediated Apoptosis Cytotoxic T lymphocytes (CTLs) recognize foreign antigens presented on the surface of infected host cells. Upon activation, CTLs secrete perforin, a transmembrane pore-forming molecule, which promotes entry of the CTL granule serine proteases called granzymes. Granzymes cleave proteins at aspaate residues and thus activate a variety of cellular caspases. In this way the CTL kills target cells by directly inducing the effector phase of apoptosis.