Which cells can directly attack virus-infected or cancerous cells in the body?

T cells, specifically cytotoxic T lymphocytes (CTLs), are the primary cells responsible for directly attacking and destroying virus-infected and cancerous cells. They accomplish this by recognizing specific antigens presented on the surface of these cells, which are typically produced as a result of viral infection or tumorigenesis. Upon recognition, T cells can engage in several mechanisms to induce apoptosis (programmed cell death) in the target cells, including the release of perforin and granzymes that cause membrane lysis and subsequent cell death.

B cells are primarily responsible for the humoral immune response, producing antibodies that can neutralize pathogens and mark them for destruction but do not directly kill infected or cancerous cells. Dendritic cells play a vital role in presenting antigens to T cells and activating them but do not directly infiltrate and kill these cells. Macrophages are crucial in phagocytizing pathogens and dead cells but typically do not engage directly with virus-infected or cancerous cells in the same targeted manner as cytotoxic T cells.