Which of the following substances is released by cytotoxic T cells and binds to a target cell's surface membrane?

• A. Interleukin 2
• B. Interferon
• C. Antibodies
• D. Perforin

Answer: (D)

Cytotoxic T cells kill target cells by releasing perforin, which binds to the surface membrane of the target and inserts into it to form pores. These pores let granzymes enter the target cell, triggering apoptosis and cell death. Perforin is a pore-forming protein specific to this lytic pathway. The other listed substances are signaling molecules or antibodies: interleukin 2 is a cytokine that promotes T cell growth, interferon has antiviral and immune-activating roles, and antibodies are produced by B cells to bind antigens. Perforin’s action on the target membrane is what directly enables the cytotoxic mechanism.