Which lymphocytes directly kill virus-infected cells?

Cytotoxic T-cells are specialized lymphocytes that play a critical role in the immune response by directly killing virus-infected cells. Upon recognizing cells that display viral antigens on their surface, cytotoxic T-cells release perforin and granzymes. Perforin creates pores in the target cell membrane, while granzymes enter through these pores to trigger a cascade of events that lead to apoptosis, or programmed cell death, in the infected cells.

This mechanism is crucial for controlling viral infections, as eliminating infected cells prevents the virus from replicating and spreading. Cytotoxic T-cells are primarily distinguished by their expression of CD8 co-receptors, which assist in their recognition of infected cells presenting antigens via MHC class I molecules.

Helper T-cells play a supportive role by enhancing the immune response but do not directly kill infected cells. B-lymphocytes are primarily responsible for producing antibodies, and natural killer cells have a similar function to cytotoxic T-cells but do not rely on specific antigen recognition to kill infected or abnormal cells. Thus, cytotoxic T-cells are the correct answer, as they are directly involved in the destruction of cells harboring viral infections.