What is the primary method used by CD8 T cells to induce apoptosis in target cells?

CD8 T cells, also known as cytotoxic T lymphocytes, primarily induce apoptosis in target cells through the exocytosis of cytotoxic granules. These granules contain potent proteins such as perforin and granzymes.

When a CD8 T cell recognizes a target cell that displays foreign antigens (such as those found on virally infected or cancerous cells), it forms a tight junction with the target, allowing it to deliver these cytotoxic granules directly. Perforin forms pores in the target cell's membrane, creating openings that enable granzymes to enter the cytosol. Granzymes are serine proteases that activate apoptotic pathways within the target cell, leading to programmed cell death.

This mechanism is highly effective and is a cornerstone of the immune response against intracellular pathogens. The direct delivery of cytotoxic agents ensures that the killing process is localized, minimizing damage to surrounding cells. In contrast, other methods listed, such as extracellular signaling and cytokine production, play roles in immune regulation and support but are not the primary means by which CD8 T cells induce cell death. Phagocytosis is more related to the action of professional phagocytes, like macrophages, rather than CD8 T cells.