Final answer:
Cytotoxic T cells or killer T cells must be activated to kill infected cells, and they do so by recognizing pathogen epitopes on MHC I molecules, followed by inducing apoptosis using perforin and granzymes.
Step-by-step explanation:
In order to kill infected cells, the type of T-cell that must be activated is the cytotoxic T cell, also known as killer T cells or TC cells. These cells target and kill cells that are infected with pathogens or are cancerous by recognizing specific pathogen epitopes presented on the cell surface using MHC I molecules.
After engagement with an antigen-presenting cell, cytotoxic T cells utilize proteins such as perforin and enzymes like granzymes to induce apoptosis in the target cells. Perforin creates pores in the target cell's membrane, allowing granzymes to enter and trigger the process of programmed cell death, effectively eliminating the infected cell without releasing the pathogens to infect nearby cells.
During an immune response, naive CD8+ T cells are activated by antigens presented on MHC II molecules by antigen-presenting cells (APCs), which then clone into activated cytotoxic T cells. The activation and proliferation of killer T cells are crucial for the adaptive immune response, particularly in protecting against viral infections where the virus is shielded from extracellular antibodies by replicating within host cells.