Which cells are primarily responsible for initiating apoptosis in target cells?

• A. B cells
• B. Macrophages
• C. T cytotoxic cells
• D. Neutrophils

Answer: (C)

T cytotoxic cells are primarily responsible for initiating apoptosis, or programmed cell death, in target cells. These cells, also known as CD8+ T cells, play a critical role in the adaptive immune response. They recognize and bind to foreign antigens presented by infected or transformed cells through Major Histocompatibility Complex (MHC) Class I molecules.

Once T cytotoxic cells identify a target cell, they release perforins and granzymes. Perforins form pores in the target cell membrane, enabling granzymes, which are serine proteases, to enter the cell. Granzymes then activate apoptotic pathways within the target cell, leading to its destruction. This mechanism is especially important in eliminating virally infected cells and tumor cells, ensuring that the integrity of the organism is maintained.

While other immune cells, such as B cells and macrophages, play crucial roles in recognizing pathogens and initiating immune responses, they do not primarily carry out the direct induction of apoptosis in target cells like T cytotoxic cells do. Neutrophils are more involved in the innate immune response and in dealing with infections primarily through phagocytosis, rather than inducing apoptosis.