Which cells destroy host cells bearing foreign antigen?

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

Answer: (B)

Direct destruction of host cells bearing foreign antigen is performed by cytotoxic T lymphocytes. These cells recognize peptides presented on MHC class I molecules by infected or abnormal cells. Upon recognition, they unleash cytotoxic granules containing perforin and granzymes that create pores and trigger programmed cell death in the target cell. They can also induce apoptosis through the Fas-FasL pathway, providing a robust mechanism to eliminate infected or transformed cells while leaving neighboring healthy cells relatively unharmed. Activation of these cytotoxic cells requires antigen presentation and co-stimulation, often aided by helper T cells providing growth signals like IL-2.

B cells are focused on eliminating extracellular pathogens by producing antibodies, not on killing host cells. Helper T cells coordinate and amplify immune responses but do not directly destroy target cells. Macrophages primarily engulf and digest pathogens and present antigens; they can kill engulfed pathogens and activate other immune cells, but their main role isn’t the direct destruction of host cells bearing foreign antigen.