Which cell type is primarily responsible for cytotoxic action against virus-infected cells?

• A. B cells
• B. Cytotoxic T cells
• C. Helper T cells
• D. Macrophages

Answer: (B)

The powerful, targeted killing of virus-infected cells comes from cytotoxic T lymphocytes. These CD8+ T cells recognize viral peptides presented on MHC class I molecules on the surface of infected cells. Once they engage their target, they release perforin and granzymes that create pores and trigger apoptosis in the infected cell, halting viral replication. This is a hallmark of the adaptive immune response, relying on antigen-specific activation and clonal expansion of these cytotoxic cells.

B cells, on the other hand, are primarily antibody producers that neutralize free virus particles rather than directly killing infected cells. Helper T cells (CD4+) coordinate the immune response, helping activate other cells, but do not themselves execute direct cytotoxic killing. Macrophages are phagocytes that ingest and destroy pathogens and present antigen to T cells, and they can contribute to defense, but they are not the main cytotoxic effectors against virus-infected host cells. Natural killer cells can kill infected cells early in infection, especially when viral evasion reduces MHC I, but the primary cytotoxic action against virus-infected cells in the classic antiviral response is carried out by cytotoxic T lymphocytes.