Which of the following is responsible for the nonspecific killing of tumor cells?

Natural killer (NK) cells play a critical role in the immune system's ability to target and destroy tumor cells in a nonspecific manner. Unlike T cells, which require specific antigens to recognize and respond to infected or cancerous cells, NK cells can identify and eliminate these cells without prior sensitization to specific antigens.

NK cells are part of the innate immune response and are particularly effective at detecting stressed, infected, or malignant cells that display abnormal markers on their surface. They operate by recognizing changes in the expression of certain proteins on target cells, such as reduced levels of major histocompatibility complex (MHC) molecules, which often occurs in tumor cells. Once identified, NK cells can induce apoptosis (programmed cell death) in these targeted cells through the release of cytotoxic granules containing perforins and granzymes.

In contrast, helper T cells primarily function to assist other immune cells and do not directly kill tumor cells. Cytotoxic T cells can attack and kill specific tumor cells but require prior sensitization and specific recognition of tumor antigens presented on MHC molecules. B cells are primarily involved in antibody production and are not responsible for direct cell killing in the context described in the question.